Methods and apparatus for performing timing synchronization with base stations

ABSTRACT

A wireless terminal using OFDM signaling supporting both terrestrial and satellite base station connectivity operates using conventional access probe signaling in a first mode of operation to establish a timing synchronized wireless link with a terrestrial base station. In a second mode of operation, used to establish a timing synchronized wireless link with a satellite base station, a slightly modified access protocol is employed. The round trip signaling time and timing ambiguity between a wireless terminal and a satellite base station is substantially greater than with a terrestrial base station. The modified access protocol uses coding of access probe signals to uniquely identify a superslot index within a beaconslot. The modified protocol uses multiple access probes with different timing offsets to further resolve timing ambiguity and allows the satellite base station access monitoring interval to remain small in duration. Terrestrial base station location/connection information is used to estimate initial timing.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.11/184,735 filed on Jul. 18, 2005, titled “METHODS AND APPARATUS FORPERFORMING TIMING SYNCHRONIZATION WITH BASE STATIONS”, which claimspriority to U.S. Provisional Patent Application Ser. No. 60/689,910,filed on Jun. 13, 2005, titled “METHODS AND APPARATUS FOR SUPPORTINGOFDM UPLINKS WITH REMOTE BASE STATIONS”, both of which is herebyexpressly incorporated by reference.

FIELD OF THE INVENTION

The present application is directed to methods and apparatus which canbe used in implementing an OFDM system which uses OFDM tones forcommunicating uplink signals to terrestrial and/or satellite basestations.

BACKGROUND

The ability to communicate using a handheld communications device, e.g.,a portable telephone, regardless of one's location in a wide area is ofgreat value. The value of such a device is important to militaryapplications as well as in the case of conventional consumer basedapplications.

Terrestrial base stations have been installed at various earth basedlocations to support voice and/or data services. Such base stationsnormally have a coverage area of a few miles at most. Accordingly, thedistance between a conventional cell phone and a base station during useis normally only a few miles. Given the relatively small distancebetween a cell phone and a terrestrial base station during normal use, ahand held cell phone normally has sufficient power to transmit to thebase station, e.g., on an uplink, using bandwidth that is relativelywide and, in many cases, capable of supporting relatively high datarates.

In the case of one known system based on the use of terrestrial basestations, a plurality of OFDM tones, e.g., in some cases 7 or moretones, are used in parallel by a wireless terminal to transmit user datato the base stations. In the known system, user data to be communicatedvia an uplink and control signals to be communicated via an uplink arenormally coded separately. In the known system, a wireless terminal maybe assigned a dedicated tone for uplink control signaling with uplinktraffic segments which correspond to tones being assigned in response toone or more uplink requests transmitted to the terrestrial base station.In the known system uplink traffic channel segment assignmentinformation is broadcast to the wireless terminals which monitorassignment signals that may indicate assignment of uplink trafficchannel segments in response to a transmitted request. On a recurringbasis, the base station of the known system also broadcasts signalswhich can be used for timing synchronization with the timingsynchronization signals, referred to as beacon signals, recurring over atime period sometimes referred to as a beacon slot.

While terrestrial base stations are useful in areas where the populationis sufficient to justify the cost of a terrestrial base station, in manylocations on the planet there is insufficient commercial justificationto deploy a base station and/or due to geographic issues it isimpractical to deploy a permanent terrestrial base station. For example,in physically inhospitable areas such as the open ocean, dessert regionsand/or regions which are covered by ice sheets it may be difficult orimpractical to deploy and maintain a terrestrial base station.

The lack of base stations in some geographic regions leads to “deadzones” in which is not possible to communicate using a cell phone. Inorder to try and eliminate the number of areas where cell phone coverageis missing, companies are likely to continue to deploy new base stationsbut, for the reasons discussed above, for the foreseeable future thereare likely to remain large areas of the planet where cell phone coveragefrom terrestrial base stations can not be obtained.

An alternative to terrestrial base stations is to use satellites as basestations. Satellite base stations are extremely costly to deploy giventhe cost of launching satellites. In addition, there is limited spaceabove the planet in which geostationary satellites can be placed. Whilesatellites in geostationary orbit have the advantage of being in a fixedposition relative to the earth, lower earth orbiting satellites can alsobe deployed but such satellites remain costly to deploy and will remainin orbit for a shorter period of time due to their initially lower orbitthan a geostationary satellite. The distance from the surface of theearth where a mobile phone may be located and geostationary orbit isconsiderable, e.g., approximately 22,226 miles although some estimatessuggest that 22,300 miles is a better estimate. To put this inperspective, the diameter of the Earth is approximately 7,926 miles.Unfortunately, the distances which signals must travel in the case ofsatellite base stations is considerable longer than the distance signalsnormally travel to reach a conventional terrestrial base station whichis usually a few miles at most.

As can be appreciated, given the distance to geostationary orbit, it isoften necessary to transmit signals to satellites at higher power levelthan is required to transmit signals to terrestrial base stations. As aresult, most satellite phones normally are relatively large and bulkycompared to conventional cell phones due to the size of the batteries,power amplifiers and other circuitry which has been used to implementcell phones. The need for a relatively large, and therefore often bulky,power amplifier results, in part, from the fact that many conventionalcommunications systems have a less than ideal peak to average powerratio. The relatively large peak to average power ratio requires that alarger amplifier be included to support peak power output than could beused in the case of the same average power output, but where the peak toaverage power ratio is lower.

Given the large distance to a satellite base station and/orcomparatively large cell size, as compared to a terrestrial basestation, uplink timing synchronization used for terrestrial basestations which use OFDM signals in the uplink may not be sufficient toachieve adequate uplink symbol timing synchronization when communicatingwith a satellite base station. Accordingly, there is a need for improvedmethods of supporting OFDM uplink signaling including improved timingsynchronization methods and/or apparatus which can be used with longround trip delays.

SUMMARY OF THE PRESENT INVENTION

The present invention is directed to communications methods andapparatus which are suitable for use in communications systems includingremote base stations and/or base stations with large coverage areas.

The methods and apparatus of the present invention can be used tosynchronize uplink transmission timing of a communications device, e.g.,a wireless terminal, with base station timing. Beacon signalstransmitted in the downlink from the base station can be used tofacilitate the timing synchronization process. A wide variety of beaconsignals can be used to support the methods and apparatus of the presentinvention. In some OFDM embodiments, beacon signals are transmitted inthe downlink using one or a few tones for one or a few consecutive timeperiods. In some embodiments beacon signals are implemented as singletone signals which are transmitted for one, two or three consecutiveOFDM symbol transmission time periods depending on the particularembodiment.

As will be discussed below, the transmission of signals bycommunications device to the base station, in OFDM systems, shouldarrive at the base station to which they are transmitted in asynchronized manner, e.g., with a synchronization level to within acyclic prefix duration in the case of OFDM symbols which are transmittedwith cyclic prefixes.

The methods and apparatus of the invention support and allow for such alevel of syncrhronization to be achieved, even with very remote basestations, through a variety of methods and techniques which can be usedalone or in combination to achieve the desired level of synchronization.While much of the discussion in the present application focuses ondownlink timing structure and beacon slots which occur in the downlink,it should be appreciated that at the base station uplink timing has afixed known relationship to downlink timing. Received signals and thetime at which signals are received at a base station can be measured interms of downlink transmission slots and downlink symbol transmissiontiming while the signals are received in the uplink.

The uplink timing structure of the present invention allows for accessintervals to occur at periodic intervals during which communicationsdevices which are not synchronized with the base station in terms ofuplink transmission timing can make access requests. Such requests maybe contention based. The base stations of the invention monitor duringthe access intervals for access requests and respond with timingcorrection and/or other information. Access intervals, while an elementof the uplink timing structure occur in a fixed known relationship todownlink timing. Each access interval normally has a duration which isless then that of a downlink superslot in duration.

Superslots, in various embodiments each include multiple OFDM symboltransmission time periods, e.g., a fixed number of OFDM symboltransmission time periods. In some, but not necessarily allimplementations, each uplink superslot includes an access interval.Access intervals in the uplink occur at fixed known locations relativeto the start of downlink superslots and beacon signals which occur inthe downlink. Accordingly, the downlink timing structure can be used asa reference for controlling uplink transmission timing as will bediscussed further below.

Numerous features of the present invention are directed to timingsynchronization. Other features of the present invention are directed tospecific access methods and apparatus which can be used to register andachieve timing synchronization with a remote base station, e.g., a basestation more than 100 miles from the location of the wireless terminal.

In various embodiments a remote base station is a base station which hasa minimum distance from a wireless terminal during use which is measuredin terms of tens, hundreds or even thousands of miles. A geostationarysatellite base station is one example of a remote base station.Geostationary satellite base stations are positioned thousands of milesabove the earth's surface in which case the minimum distance to acommunications device on the earth's surface or even in a commercialairplane is measured in thousands of miles. This is in contrast to anear base station which might be a terrestrial base station locatedwithin, e.g., up to 50 miles of a wireless terminal during normal usebut more typically up to 5 miles.

While the methods and apparatus of the present invention, including thecell phones of the present invention are well suited for use incommunications systems which have both terrestrial and satellite basestations, the methods and apparatus of the present invention are wellsuited for a wide range of communications applications where a largedifference in the amount of output power for a fixed amount of bandwidthis required. In the satellite example, it should be appreciated that afar greater amount of output power for a fixed amount of bandwidth isnormally required for successful uplink signaling to the satellite basestation than is required for successful uplink signaling using the sameamount of transmission bandwidth to a terrestrial base station.

Various features of the present invention are directed to methods andapparatus which can be used to implement portable communications devicescapable of communicating with both remote and comparatively near basestations, e.g., satellite base stations and terrestrial base stations. Asystem implemented in accordance with the invention may include aplurality of near and remote base stations. In one such system,terrestrial base stations are used to provide communications coveragewith sufficient communications traffic to justify the deployment of aterrestrial base station. Satellite base stations are used to providefill in coverage in regions where terrestrial base stations are notdeployed, e.g., due to the nature of the physical environment, the lackof a site for a base station or for other reasons. Portablecommunications devices in the exemplary system are capable ofcommunicating with both the terrestrial and satellite base stations,e.g., by switching between different modes of operation.

As will be discussed below, in various embodiments, the system isimplemented as an OFDM system. In some embodiments, OFDM signaling isused for uplink as well as downlink signaling. First and second modes ofOFDM uplink operation are supported.

During normal operation with terrestrial base stations, the wirelessterminal uses multiple tones in parallel in the uplink to transmit userdata on multiple tones to a base station simultaneously. This allowsrelatively high data rates to be supported. When operating in multi-tonemode, the average peak to average power ratio, during portions of timein which user data is transmitted on multiple tones, is a first ratio.As will be discussed below, when operating in a single tone mode ofoperation, e.g., used for communicating with a satellite base station, asecond, lower peak to average power ratio is achieved. Thus, whenoperating in the single tone mode, the power amplifier can be used in amore efficient manner. In various embodiments, the difference is 4 ormore db, and commonly 6 db, in the peak to average power ratio betweenthe multi-tone mode of operation and the single tone mode of operationwhich is achieved for a period of several symbol times.

Single-tone-mode is a method of operating an OFDM wireless terminal tomaximize its uplink power budget coverage under typical powerconstraints encountered when communicating with terrestrial basestations. This mode is suitable for low rate data of voice links inwhich multi-tone channels, ACKs are not supported.

In single tone mode the terminal will transmit on an OFDM single tone ata time. This tone is represented as a single, constant logical tone;however, it can, and in various embodiments does, hop from physical toneto physical tone on dwell boundaries as consistent with other OFDMchannels used in some systems. In one embodiment, this logical tonereplaces a UL-DCCH channel used to communicate with a terrestrial basestation thus maintaining compatibility with other OFDM users operatingin standard multi-tone mode.

The contents of the single tone uplink channel used by a wirelessterminal includes, in some embodiments, a multiplex of control data anduser data. This multiplex may be at the field level within a code word,i.e. some bits from a channel coding block are used to represent controldata the remainder represent user data. However in other embodiments themultiplexing in the single tone uplink channel is at the code wordlevel, e.g., control data is coded within a channel coding block, userdata is coded within a channel coding block, and the blocks aremultiplexed together for transmission in the single tone uplink channel.In one embodiment, when the single tone channel is not fully occupiedwith user data (e.g., during silence suppression of a voice call) it ispossible to blank the transmitter during the un-need transmit symbolsthereby conserving transmitter power since no signals need be sentduring this period. User data may be multiplexed packet data orregularly scheduled voice data, or a mix of the two.

For a terminal operating in single-tone mode, downlink acknowledgementsignals can not be transmitted in a separate channel as is done in themulti-tone mode and thus downlink acknowledgements are eithermultiplexed into the logical single tone uplink channel tone, or ACKsare not used. In such a case, the base station may assume that downlinktraffic channel segments have been successfully received with thewireless terminal expressly requesting retransmission if needed.

In accordance with the invention, a wireless terminal operating insingle tone mode can achieve a benefit in transmitted power while usingstandard OFDM components to implement the transmitter. In standard mode,the average power transmitted is normally limited below the peak powercapacity of the transmitter's power amp to allow for peak-to-averageratio (PAR), typically 9 dB, and avoid peak clipping which can causeexcessive out-of-band emission. In single tone mode, in variousembodiments, the PAR is limited to approximately 3 dB thus the averagetransmit power can be increased by almost 6 dB without increasing theprobability of clipping.

At frequency hops (changes in the physical tone corresponding to thesingle logical tone occur at dwell boundaries), the phase of thetransmitted waveform can be controlled to as to be phase continuousacross frequencies. This can, and is accomplished in some but notnecessarily all embodiments by changing the carrier frequency of thetone during the cyclic extension of the OFDM symbol from one symboltransmitted in the uplink to the next so that the signal phase at theend of the symbol is at a desired value equal to the starting phase ofthe subsequent symbol. This phase continuous operation will allow thePAR of the signal is bounded at 3 dB.

OFDM over geo-stationary satellite is possible with a few modificationsof the basic existing basic OFDM communications protocols. Due to theextremely long round-trip time (RTT) there is little or no value ofslaved acknowledgments for traffic channels.

Thus, in some embodiments of the invention, when operating in singletone uplink mode downlink acknowledgment are not sent. In some suchembodiments, downlink acknowledgements are replaced with a repeatrequest mechanism in which a request is transmitted in the UL for arepeat transmission of the data which was not received successfully.

Numerous features, benefits and embodiments of the present invention arediscussed in the detailed description which follows.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a drawing of an exemplary wireless communications systemimplemented in accordance with the present invention and using methodsof the present invention.

FIG. 2 is a drawing of an exemplary base station, e.g., a terrestrialbased base station, implemented in accordance with the present inventionand using methods of the present invention.

FIG. 2A is a drawing of an exemplary base station, e.g., a satellitebased base station, implemented in accordance with the present inventionand using methods of the present invention.

FIG. 3 is a drawing of an exemplary wireless terminal, e.g., mobilenode, implemented in accordance in the present invention and usingmethods of the present invention.

FIG. 4 is a drawing illustrating exemplary uplink information bitencoding for an exemplary WT, e.g., MN, operating in a single-toneuplink mode of operation, in accordance with various embodiments of thepresent invention.

FIG. 5 is a drawing illustrating an exemplary OFDM wireless multipleaccess communications system including a hybrid of base stations thatare both terrestrial based and space based, in accordance with variousembodiments of the present invention.

FIG. 6 is a drawing showing exemplary backhaul interconnectivity betweenthe various satellite based and terrestrial based base stations of FIG.5.

FIG. 7 is a flowchart of an exemplary method of operating a wirelessterminal, e.g., mobile node, in accordance with the present invention.

FIG. 7A is a drawing illustrating relatively long round trip signalingtimes and significantly different signal path lengths between anexemplary satellite base station and different mobile nodes located atdifferent points in the satellite base station's cellular coverage areaon the surface of earth, resulting in timing synchronizationconsiderations, which are addressed in accordance with methods andapparatus of the present invention.

FIG. 8 illustrates an exemplary hybrid system including both terrestrialand satellite based base stations and a wireless terminal utilizingterrestrial base station location information to reduce round triptiming ambiguity with respect to a satellite base station.

FIG. 8A illustrates an embodiment of where multiple terrestrial basestations are associated with the same satellite base station coveragearea, and terrestrial base station location and/or connectioninformation is used to reduce WT/satellite base station timingambiguity, in accordance with the present invention.

FIG. 9 is a drawing illustrating that in an exemplarysatellite/terrestrial hybrid wireless communication system the roundtrip signal delay between a satellite base station and a terrestriallylocated wireless terminal will be greater than a typical superslot timeinterval used in some terrestrial based wireless communications systems.

FIG. 10 is a drawing illustrating the feature of coding an access probesignal with information identifying a relative time interval value,e.g., a superslot index value, within a larger relative time interval,e.g., a beacon slot, within the timing structure, said coded informationbeing used in the access process to determine timing synchronizationbetween the satellite base station and the WT, in accordance with thepresent invention.

FIG. 11 is a drawing illustrating a feature of using multiple accessprobe signals, with different timing offsets such that the timingsynchronization between the satellite base station and the WT can befurther resolved to within a smaller time interval, in accordance withthe present invention.

FIG. 12 further illustrates the concept of a wireless terminal sendingmultiple access probes to the satellite base station with differenttiming offsets, in accordance with the present invention.

FIG. 13 is a drawing illustrating exemplary access signaling inaccordance with methods of the present invention.

FIG. 14 is a drawing illustrating exemplary access signaling inaccordance with methods of the present invention.

FIG. 15 is a drawing illustrating exemplary access signaling inaccordance with methods of the present invention.

FIG. 16 comprising the combination of FIG. 16 and FIG. 16B is aflowchart of an exemplary method of operating a wireless terminal toaccess a base station and perform a timing synchronization operation inaccordance with the present invention.

FIG. 17 comprising the combination of FIG. 17A and FIG. 17B is aflowchart of an exemplary method of operating a communications devicefor use in a communications system.

FIG. 18 is a flowchart of an exemplary method of operating an exemplarycommunications device in accordance with the present invention.

FIG. 19 is a flowchart of an exemplary method of operating an exemplarycommunications device in accordance with the present invention.

FIG. 20 is a flowchart of an exemplary method of operating a wirelesscommunications terminal in a system in accordance with the presentinvention.

FIG. 21 is a drawing of an exemplary wireless terminal, e.g., mobilenode, implemented in accordance with the present invention.

FIG. 22 is a flowchart of an exemplary method of operating a basestation in accordance with the present invention.

FIG. 23 is a drawing of an exemplary wireless terminal, e.g., mobilenode, implemented in accordance with the present invention.

FIG. 24 is a drawing of an exemplary wireless terminal, e.g., mobilenode, implemented in accordance with the present invention.

FIG. 25 is a drawing of an exemplary base station implemented inaccordance with the present invention and using methods of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 is a drawing of an exemplary wireless communications system 100implemented in accordance with the present invention and using methodsof the present invention. The exemplary system 100 is an exemplaryOrthogonal Frequency Division Multiplexing (OFDM) multiple access spreadspectrum wireless communications system. The exemplary system 100includes a plurality of base stations (102, 104) and a plurality ofwireless terminals (106, 108), e.g., mobile nodes. The various basestations (102, 104) may be coupled together via a backhaul network. Themobile nodes (NM1 106, MN N 108) may move throughout the system and usea base station, in whose coverage area it is currently located, as itpoint of network attachment. Some of the base stations are terrestrialbased base stations, e.g., BS 102, and some of the base stations aresatellite based base stations, e.g., BS 104. From the perspective of theMNs (106, 108), the terrestrial base stations are considered nearby basestations (102) while the satellite based base stations are consideredremote base stations (104). The MNs (106, 108) include the capability tooperate in two different modes of operation, e.g., an uplink multi-tonemode of operation tailored to the power and timing considerations ofcommunicating with a nearby, e.g., terrestrial base 'station 102 and anuplink single tone mode of operation tailored to the power and timingconsiderations of communicating with a remote, e.g., satellite, basestation 104. At some times, MN1 106 may be coupled to the satellite BS104 via wireless link 114 and may be operating in an uplink single tonemode of operation. At other times, MN1 106 may be coupled to theterrestrial base station 102 via wireless link 110 and may be operatingin a more conventional multi-tone uplink mode of operation. Similarly,at some times, MN N 108 may be coupled to the satellite BS 104 viawireless link 116 and may be operating in an uplink single tone mode ofoperation. At other times, MN N 108 may be coupled to the terrestrialbase station 102 via wireless link 112 and may be operating in a moreconventional multi-tone uplink mode of operation.

Other MNs may exist in the system that support communications with onetype of base station, e.g., a terrestrial base station 102, but do notsupport communications with the other type of base station, e.g., thesatellite base station 104.

FIG. 2 is a drawing of an exemplary base station 200, e.g., aterrestrial based base station, implemented in accordance with thepresent invention and using methods of the present invention. Exemplarybase station 200 may be the nearby, e.g., terrestrial, base station 102of the exemplary system 100 of FIG. 1. The base station 200 is sometimesreferred to an access node, as the base station 200 provides networkaccess to WTs. The base station 200 includes a receiver 202, atransmitter 204, a processor 206, an I/O interface 208, and a memory 210coupled together via a bus 212 over which the various elements mayinterchange data and information. The receiver 202 includes a decoder214 for decoding received uplink signals from WTs. The transmitter 204includes an encoder 216 for encoding downlink signals to be transmittedto WTs. The receiver 202 and transmitter 204 are each coupled toantennas (218, 220) over which uplink signals are received from WTs anddownlink signals are transmitted to WTs, respectively. In someembodiments, the same antenna is used for receiver 202 and transmitter204. The I/O interface 208 couples the base station 200 to theInternet/other network nodes. The memory 210 includes routines 222 andata/information 224. The processor 206, e.g., a CPU, executes theroutines 222 and uses data/information 224 in memory 210 to control theoperation of the base station 200 and implement the methods of thepresent invention. Routines 222 include a communications routine 226 andbase station control routine 228. The communications routine 226implements the various communications protocols used by the base station200. The base station control routine 228 includes a scheduler module230, which assigns uplink and downlink segments to WTs including uplinktraffic channel segments, downlink control modules 232 and uplinkmulti-tone user control modules 234. Downlink control module 232controls downlink signaling to WTs including beacon signaling, pilotsignaling, assignment signaling, downlink traffic channel segmentsignaling, and automatic retransmission mechanisms regarding downlinktraffic channel segments in accordance with acks/naks received. Uplinkmulti-tone user control modules 234 control operations related to a WToperating in multi-tone uplink mode, e.g., access operations, operationsof receiving and processing uplink traffic channel user data from a WTcommunicated over multiple, e.g., 7, tones simultaneously in an assigneduplink traffic channel segment, with assignment changing betweendifferent WTs over time, timing synchronization operations, andprocessing of control information from a WT communicated over adedicated control channel using a dedicated logical tone.

Data/information 224 includes user data/information 236 which includes aplurality of sets of information (user 1/MN session A session Bdata/information 238, user N/MN session X data/information 240)corresponding to the wireless terminals using the base station 200 astheir point of network attachment. Such WT user data/information mayinclude, e.g., WT identifiers, routing information, segment assignmentinformation, user data/information, e.g., voice information, datapackets of text, video, music, etc., coded blocks of information.Data/information 224 also includes system information 242 includingmulti-tone UL user frequency/timing/power/tone hopping/coding structureinformation 244.

FIG. 2A is a drawing of an exemplary base station 300, e.g., a satellitebased base station, implemented in accordance with the present inventionand using methods of the present invention. Exemplary base station 300may be BS 104 of exemplary system 100 of FIG. 1. The base station 300 issometimes referred to an access node, as the base station providesnetwork access to WTs. The base station 300 includes a receiver 302, atransmitter 304, a processor 306, and a memory 308 coupled together viaa bus 310 over which the various elements may interchange data andinformation. The receiver 302 includes a decoder 312 for decodingreceived uplink signals from WTs. The transmitter 304 includes anencoder 314 for encoding downlink signals to be transmitted to WTs. Thereceiver 302 and transmitter 304 are each coupled to antennas (316, 318)over which uplink signals are received from WTs and downlink signals aretransmitted to WTs, respectively. In some embodiments, the same antennais used for the receiver 302 and transmitter 304. In addition tocommunicating with WTs, the base station 300 can communicate with othernetwork nodes, e.g., a ground station with a directional antenna andhigh capacity link, the ground station coupled to other network nodes,e.g., other base stations, routers, AAA servers, home agent nodes andthe Internet. In some embodiments, the same receivers 302, transmitters304, and/or antennas previously described with BS-WT communication linksare used for BS-network node ground station links, while in otherembodiments separate elements are used for different functions. Thememory 308 includes routines 320 and data/information 322. The processor306, e.g., a CPU, executes the routines 320 and uses thedata/information 322 in memory 308 to control the operation of the basestation 300 and implement the methods of the present invention. Thememory 308 includes a communications routine 324 and base stationcontrol routine 326. The communications routine 324 implements thevarious communications protocols used by the base station 300. The basestation control routine 326 includes a scheduler module 328, whichassigns downlink segments to WTs and reschedules downlink segments toWTs in response to received requests for retransmission, downlinkcontrol modules 330, single uplink tone user control modules 332, andnetwork module 344. Downlink control module 330 controls downlinksignaling to WTs including beacon signaling, pilot signaling, downlinksegment assignment signaling, and downlink traffic channel segmentsignaling. The single UL tone user control modules 332 performoperations including: assigning a single dedicated logical tone to a WTuser to be used for uplink signaling including both user data andcontrol information and timing synchronization operations with a WTseeking to use the BS as its point of network attachment. Network module334 controls operations related to the I/O interface with the networknode ground station link.

Data/information 322 includes user data/information 336 which includes aplurality of sets of information (user 1/MN session A session Bdata/information 338, user N/MN session X data/information 340)corresponding the wireless terminals using the base station 300 as theirpoint of network attachment. Such WT information may include, e.g., WTidentifiers, routing information, assigned uplink single logical tone,downlink segment assignment information, user data/information, e.g.,voice information, data packets of text, video, music, etc., codedblocks of information. Data/information 322 also includes systeminformation 342 including single-tone UL userfrequency/timing/power/tone hopping/coding structure information 344.

FIG. 3 is a drawing of an exemplary wireless terminal 400, e.g., mobilenode, implemented in accordance in the present invention and usingmethods of the present invention. Exemplary WT 400 may be any of the MNs106, 108 of the exemplary system 100 of FIG. 1. The exemplary wirelessterminal 400 includes a receiver 402, a transmitter 404, a processor406, and memory 408 coupled together via a bus 410 over which thevarious elements may interchange data/information. The receiver 402,coupled to a receive antenna 412, includes a decoder 414 for decodingdownlink signals received from BSs. The transmitter 404 coupled, to atransmit antenna 416, includes an encoder 418 for encoding uplinksignals being transmitted to BSs. In some embodiments, the same antennais used for the receiver 402 and transmitter 404. In some embodiments,an omni-directional antenna is used.

The transmitter 404 also includes a power amplifier 405. The same poweramplifier 405 is used by the WT 400 for both the multi-tone uplink modeof operation and the single tone uplink mode of operation. For example,in the multi-mode uplink operational mode, where the uplink trafficchannel segments may typically use 7, 14, or 28 tones simultaneously,the power amplifier needs to accommodate peak conditions where the 28signals corresponding to the 28 tones simultaneously constructivelyalign, this tends to limit the average output level. However, when theWT 400 is operated in a single uplink tone mode of operation, using thesame power amplifier, the concern constructive alignment between signalsfrom different tones is not an issue, and the average power output levelfor the amplifier can be considerably increased over the multi-toneoperational mode. This approach, in accordance with the presentinvention, allows for a conventional terrestrial mobile node, to beadapted, with minor modifications, and used to communicate uplinksignals to a satellite base station at a substantially increaseddistance.

The memory 408 includes routines 420 and data/information 422. Theprocessor 406, e.g., a CPU, executes the routines 420 and uses thedata/information 422 in memory 408 to control the operation of thewireless terminal 400 and implement the methods of the presentinvention. The routines 420 include a communications routine 424 andwireless terminal control routines 426. The communications routine 424implements the various communications protocols used by the wirelessterminal 400. The wireless terminal control routines 426 include aninitialization module 427, a handoff module 428, an uplink modeswitching control module 430, uplink single tone mode module 432, uplinkmulti-tone mode module 434, an uplink tone hopping module 436, a codingmodule 438, and a modulation module 440.

The initialization module 427 controls operations regarding start-up ofthe wireless terminal, e.g., including start-up from a power off to apower on state of operation, and operations related to the wirelessterminal 400 seeking to establish a wireless communications link with abase station. The handoff module 428 controls operations related tohandoffs form one base station to another, e.g., the WT 400 may becurrently connected with a terrestrial base station, but be involved ina handoff to a satellite base station. Uplink switching control module430 controls switching between different modes of operation, e.g.,switching from a multi-tone uplink mode of operation to a single toneuplink mode of operation when the wireless terminal switches fromcommunicating with a terrestrial base station to a satellite basestation. Uplink single tone mode module 432 includes modules used in thesingle tone mode of operation with satellite base stations, while ULmulti-tone mode module 434 includes modules used in the multi-tone modeof operation with terrestrial base stations.

Uplink single tone mode module 432 includes a user data transmissioncontrol module 442, a transmission power control module 444, a controlsignaling module 446, a UL single tone determination module 448, acontrol data/user data multiplexing module 450, a DL traffic channelretransmission request module 452, a dwell boundary and/or inter-symbolboundary carrier adjustment module 454, and an access module 456. Theuser data transmission module 442 controls operations related to uplinkuser data while in the single tone mode of operation. The transmissionpower control module 444 controls the transmission of power during thesingle tone uplink mode to maintain an average peak to average powerratio which is at least 4 dB lower than a peak to average power ratiomaintained during said multi-tone uplink mode of operation. The controlsignaling module 446 controls signaling during the single tone mode ofoperation, and such control operations include reducing the frequencyand/or number of the uplink control signals which are transmitted fromthe WT 400 when operation switches from the multi-tone mode of operationto the single tone mode of operation. The uplink single tonedetermination module 448 determines the single logical tone in theuplink timing structure which has been assigned to the WT to be used foruplink signaling, e.g., via an association with a base station assignedWT identifier. The control data/user data multiplexing module 450multiplexes user data information bits with control data bits providinga combined input that may be coded as a block. The downlink trafficchannel retransmission request module 452 issues requests forretransmission of downlink traffic channel segment which were notsuccessfully decoded, e.g., provided the WT deems the data would stillbe valid given the large delay involved due to the long round tripsignaling time. Dwell boundary carrier adjustment module 454 slightlychanges the carrier frequency of the tone during the cyclic extension ofthe OFDM symbol that terminates a dwell so that the signal phase at theend of the symbol is at a desired value equal to the starting phases ofthe subsequent symbol. In this way, in accordance with a feature of someembodiments of the present invention, at frequency hops, the phase ofthe transmitted waveform can be controlled to be phase continuous acrossfrequencies. In some embodiments, the frequency adjustment is performed,e.g., as part of a multi-part cyclic prefix included in each ofsuccessive OFDM symbols, to provide continuity between successive uplinkOFDM symbols transmitted by the WT over the uplink during the single ULtone mode of operation. This continuity between symbols of the signal isadvantageous in maintaining peak power level control, which affects thelevel to which the power amplifier 405 can be driven while in the singletone mode of operation.

The access module 456 controls operations related to establishing a newwireless link with a satellite base station. Such operation may include,e.g., timing synchronization operations including access probe signalingin accordance with various features of some embodiments of the presentinvention. For geo-stationary satellites with a beam covering a largegeographical area there may be significant differences in the round triptime between the center of the beam and the edge. To resolve this RTTambiguity, a ranging scheme capable of resolving delta-RTT of severalmilliseconds is used. For example, the timing structure may be dividedinto different time segments, such as, e.g., superslots, where asuperslot represents 114 successive OFDM symbol transmission timeintervals, and different coding of the access probe signal may be usedfor different superslots. This can be used to allow timing ambiguitybetween the WT and satellite BS to be resolved to within a superslot. Inaddition, repeated access attempts at various time offsets can beattempted repeatedly to cover the superslot ambiguity, e.g., (<11.4msec). In some embodiments, position about the last terrestrial BSdetected can be used to form an initial round trip time estimate (WT-SATBS-WT) and this estimate can compress the range used to within the rangesupported by access signaling typically used with terrestrial basestations.

The uplink multi-tone module 434 includes a user data transmissioncontrol module 458, a transmission power control module 460, a controlsignaling module 462, an uplink traffic channel request module 464, anuplink traffic channel tone set determination module 466, an uplinktraffic channel modulation/coding selection module 468, a downlinktraffic channel ack/nak module 470, and an access module 472. The userdata transmission control module 458 includes operations includingcontrolling transmission of uplink traffic channel segments assigned tothe WT.

User data transmission control module 458 controls uplink transmissionrelated operations of user data in the multi-tone mode of operation,wherein user data is communicated in an uplink traffic channel segment,temporarily assigned to the WT, and including signals to be transmittedusing multiple tones simultaneously. Transmission power control module460 controls uplink transmission power levels in the multi-tone mode ofuplink operation, e.g., adjusting output power levels in accordance withreceived base station uplink power control signals and within thecapabilities of the power amplifier, e.g., in terms of not exceedingpeak power output capability of power amplifier. Control signalingmodule 462 controls power and timing control signaling operations whilein the multi-tone uplink mode of operation, the rate of controlsignaling being higher than in the single-tone uplink mode of operation.In some embodiments, control signaling module 462 includes the use of adedicated control channel logical tone dedicated to the WT by the BS,e.g., corresponding to a BS assigned WT identifier, for use in uplinkcontrol signaling. Control signaling module 462 may code controlinformation for transmission in uplink control channel segments which donot include user data. UL traffic channel request module 464 generatesrequests for traffic channel segments to be assigned, e.g., when the WT400 has user data to communicate on the uplink. UL traffic channel toneset determination module 466 determines the set of tones to usecorresponding to an assigned uplink traffic channel segment. The set oftones includes multiple tones to be used simultaneously. In themulti-tone mode of operation, the logical tone set assigned to a WT forcommunicating uplink traffic channel user data at one time may differfrom the logical tone set assigned to the WT for communicating uplinktraffic channel user data at a different time, even though the WT mayhave been assigned the same WT identifier by the same BS. Module 466 canalso use tone hopping information to determine the physical tonescorresponding to the logical tones. UL traffic channel modulation/codingselection module 468 selects and implements the uplink coding rate andmodulation method to be used for an uplink traffic channel segment. Forexample, in the UL multi-tone mode, the WT may support a plurality ofuser data rates implemented using different coding rates and/ordifferent modulation methods, e.g., QPSK, QAM 16. DL traffic channelAck/Nak module 470 controls Ack/Nak determination and response signalingof received downlink traffic channel segments, while in the uplinkmulti-tone mode of operation. For example, for each downlink trafficchannel segment in the downlink timing structure, there may be acorresponding Ack/Nak uplink segment in the uplink timing structure forthe UL multi-tone mode of operation, and the WT, if assigned thedownlink traffic channel segment sends an Ack/Nak back to the BSconveying the result of the transmission, e.g., to be used in anautomatic retransmission mechanism. Access module 472 controls accessoperations while in the multi-tone mode of operation, e.g., accessoperations to establish a wireless link with a nearby, e.g., terrestrialbase station, and achieve timing synchronization. In some embodiments,the access module 472 for multi-tone mode has a lower level ofcomplexity than the access module 456 for single-tone mode.

Data/information 422 includes uplink operational mode 474, base stationidentifier 476, base stations system information 475, base stationassigned wireless terminal identifier 477, user/device/session/resourceinformation 478, uplink user voice data information bits 479, uplinkuser multiplexed packet data information bits 480, uplink control datainformation bits 481, coded block including uplink user data and controldata 482, coded user data block, coded control data block 484, frequencyand timing structure information 485, single tone mode coding blockinformation 488, multi-tone mode coding block information 489, singletone mode transmitter blanking criteria/information 490, single tonemode transmitter power adjustment information 491, multi-tone modetransmitter power adjustment information 492, and single tone modecarrier frequency/cyclic extension adjustment information 493. Theuplink operational mode 474 includes information identifying whether theWT 400 is currently in the multi-tone uplink mode, e.g., forcommunications with a terrestrial base station or in the single-toneuplink mode, e.g., for communications with a satellite base station. BSssystem information 475 includes information associated with each of thebase stations in the system, e.g., type of base station satellite orterrestrial, carrier frequency or frequencies used by the base station,base station identifier information, sectors in the base station, timingand frequency uplink and downlink structures used by the base station,etc.

BS identifier 476 includes an identifier of the BS the WT 400 is usingas its current point of network attachment, e.g., distinguishing the BSfrom other BSs in the overall system. BS assigned WT identifier 477 maybe an identifier, e.g., a value in the range 0 . . . 31, assigned by theBS being used as the WTs point of network attachment. In the singletone-tone uplink mode of operation, the identifier 477 may be associatedwith a single dedicated logical tone in the uplink timing structure tobe used by the WT for uplink signaling including both user data andcontrol data. In the multi-tone uplink mode of operation, the identifier477 may be associated with a logical tone in the uplink timing structureto be used by the WT for a dedicated control channel for uplink controldata. The BS assigned WT identifier 477 may also be used by the BS whenmaking segment assignments, e.g., of an uplink traffic channel segmentin the multi-tone mode of uplink operation.

User/device session/resource information 478 includes user and deviceidentification information, routing information, security information,ongoing session information, and air link resource information. Uplinkuser voice data information bits 479 include input user datacorresponding to a voice call. Uplink user multiplexed packet datainformation bits 480 includes input user data, e.g., corresponding totext, video, music, a data file, etc. Uplink control data informationbits 481 includes power and timing control information that the WT 400desires to communicate to the BS. Coded block including uplink user dataand control bits 482 is the coded output block corresponding to amixture of user information bits 478 and/or 479 in combination withcontrol information bits 481, which is formed in some embodiments duringthe UL single tone mode of operation. Coded user data block 483 is acoded block of user information bits 478 and/or 479, while coded controldata block 484 is a coded block of control information bits 481. Dataand control information are coded separately in the UL multi-tone modeof operation, and in some embodiments, of the UL single tone mode ofoperation. In some embodiments of the single-tone mode of operationwhere coding between uplink user data and uplink control data isseparate, the ability to blank the transmitter, when there is no userdata to communicate, is facilitated. Single tone mode transmitterblanking criteria/information 490 is used in the blanking decisions,e.g., applying no output transmitter power on the single uplink toneduring some intervals dedicated to user data, where there is no data tocommunicate, e.g., due to a lull in an ongoing conversation. Thisapproach of transmitter blanking results in power saving for thewireless terminal, an important considerations where the average poweroutput is relatively high to facilitate communications with a satellitein geo-stationary orbit. In addition, levels of interference may bereduced.

Single tone mode coding block information 488 includes informationidentifying the coding rate and modulation method used for the uplink inthe single tone mode of operation, e.g., a low coding rate using QPSKmodulation, e.g., supporting at least 4.8 KBits/sec. Multi-tone modecoding block information 489 includes a plurality of different data rateoptions that are supported for uplink traffic channel segments in theuplink during the multi-tone mode of operation, e.g., various codingrates and modulation schemes including QAM4, e.g., QPSK, and QAM16, suchas to support at least the same coding rate as in the single tone modeplus some additional higher data rates.

Frequency and timing structure information 485 includes dwell boundaryinformation 486 and tone hopping information 487, corresponding to theBS being used as the point of network attachment. Frequency and timingstructure information 485 also includes information identifying logicaltones within the timing and frequency structure.

Single tone mode transmitter power adjustment information 491 andmulti-tone mode power adjustment information 492 includes informationsuch as peak power, average power, peak to average power ratio, maximumpower levels, for operation and control of the power amplifier 405, whenin the single tone mode and multi-tone mode of operation, respectively.Single tone mode carrier frequency cyclic extension adjustmentinformation 493 includes information used by the dwell boundary and/orinter-symbol boundary carrier adjustment module 454 to implementcontinuity between signals at symbol boundaries in the uplink during thesingle tone mode of operation, e.g., especially during hops at a dwellboundary from one physical tone to another.

FIG. 4 is a drawing 500 illustrating exemplary uplink information bitencoding for an exemplary WT, e.g., MN, operating in a single-toneuplink mode of operation, in accordance with various embodiments of thepresent invention. A logical tone, in the uplink frequency structure, isassigned directly or indirectly, e.g., by the base station, to the WT.For example, the BS may assign the single-tone mode WT a user identifierthat may be associated with a specific dedicated logical tone. Forexample, the logical tone may be the same logical tone used as adedicated control channel (DCCH) tone, if the WT is in a multi-tone modeof operation, e.g., where the WT normally communicates uplink trafficchannel information using seven or more tones at the same time. Thelogical tone may be mapped to a physical tone in accordance with tonehopping information known to both the base station and the WT. Tonehopping between different physical tones may occur on dwell boundaries,where a dwell may be a fixed number, e.g., seven, of consecutive OFDMsymbol transmission time intervals in a timing structure used in theuplink. The same logical tone in the uplink frequency structure is usedin the single-tone mode of operation to convey both control informationbits 502 and user data information bits 504. The control informationbits 502 may include, e.g., power and timing control information. Theuser data bits 504 may include voice user data information bits 506and/or multiplexed packet user data bits 508. A multiplexer 510 is usedto receive the control data information bits 502 and the user datainformation bits 504. The output 512 of the multiplexer 510 is an inputto an uplink block encoding module 514 which encodes the combination ofcontrol and user information bits and outputs a coded block of codedbits 516. The coded bits are mapped onto modulation symbols, inaccordance with the uplink modulation scheme used, e.g., a low rate QSPKmodulation scheme, and the modulation symbols are transmitted using thephysical tone corresponding to the assigned logical tone. The uplinkrate is such as to support at least one single voice call. In someembodiments, the uplink user information rate is at least 4.8 Kbits/sec.

FIG. 5 is a drawing illustrating an exemplary OFDM wireless multipleaccess communications system 600 including a hybrid of base stationsthat are both terrestrial based and space based, in accordance withvarious embodiments of the present invention. Each satellite (satellite1 602, satellite 2 604, satellite N 606) includes a base station(satellite base station 1 608, satellite base station 2 610, satellitebase station N 612), implemented in accordance with the presentinvention and using methods of the present invention. The satellites(602, 604, 608) may be, e.g., geo-stationary satellites, located inspace 601 in a high earth orbit of approximately 22,300 mi around theequator of the earth 603. The satellites (602, 604, 606) may havecorresponding cellular coverage areas on the surface of the earth (cell1 614, cell 2 616, cell N 618), respectively. The exemplary hybridcommunications system 600 also includes a plurality of terrestrial basestation (terrestrial BS 1′ 620, terrestrial BS 2′ 622, terrestrial BS N′624), each with a corresponding cellular coverage area (cell 1′ 626,cell 2′ 628, cell N′ 630), respectively. Different cells or portions ofdifferent cell may or may not overlap with one another either partiallyor completely. Typically, the size of a terrestrial base stations cellis smaller than the size of a satellite's cell. Typically, the number ofterrestrial base stations exceeds the number of satellite base stations.In some embodiments, many relatively small terrestrial BS cell arelocated within a satellites relatively large cell. For example, in someembodiments, terrestrial cells have a typical radius of 1-5 mi, whilesatellite cells typically have a radius of 100-500 mi. A plurality ofwireless terminals, e.g., user communications devices such as cellphones, PDA, data terminals, etc., implemented in accordance with thepresent invention and using methods of the present invention exist inthe system. The set of wireless terminals may include stationary nodesand mobile nodes; the mobile nodes may move throughout the system. Amobile node may use a base station, in whose cell it currently resides,as its point of network attachment. In some embodiments, the terrestrialBSs are used by the WTs as the default type of base station to first tryto use in locations where access could be provided by either aterrestrial or satellite base station, with the satellite base stationsbeing used primarily to provide access in those areas not covered by aterrestrial base station. For example, in some areas it may beimpractical to install a terrestrial base station for economic,environmental, and/or terrain reasons, e.g., due to low populationdensity, due to rugged inhospitable terrain, etc. In some terrestrialbase station cells, there may be dead spots, e.g., due to obstructionssuch as mountains, high buildings, etc. In such dead spot locationssatellite base stations could be used to fill in the gaps in coverage toprovide the WT user with more seamless overall coverage. In addition,priority considerations, and user subscribed tier levels are used, insome embodiments, to determine access to satellite base stations. Thebase stations are coupled together, e.g., via a backhaul network,providing interconnectivity for the MNs located in different cells.

MNs communicating with a satellite base station may be operating in asingle-tone mode of operation where a single tone is used for theuplink, e.g., supporting a voice channel. In the downlink, a larger setof tones may be used, e.g., 113 downlink tones, which are received andprocessed by the WT. For example, in the downlink the WT may be assignedtemporarily, as needed, a downlink traffic channel segment using aplurality of tones simultaneously. In addition, the WT may receivecontrol signaling simultaneously over different tones. Cell 1 614includes (MN1 632, MN N 634) communicating with satellite BS 1 608 viawireless links (644, 646), respectively. Cell 2 616 includes (MN1′ 636,MN N′ 638) communicating with satellite BS 2 610 via wireless links(648, 650), respectively. Cell N 618 includes (MN1″ 640, MN N′ 642)communicating with satellite BS N 612 via wireless links (652, 654),respectively. In some embodiments, the downlink between the satellite BSand the MN supports a higher rate of user information than thecorresponding uplink, e.g., supporting voice, data, and/or digital videobroadcast in the downlink. In some embodiments, the downlink user datarate provided a WT, using a satellite BS as its point of networkattachment, is approximately the same as the uplink user data rate,e.g., 4.8 Kbit/sec, thus supporting a single voice call, but tending toconserve power resources of the satellite base station.

MNs communicating with a terrestrial base stations may be operating in aconventional mode of operation, e.g., where multiple tones, e.g., sevenor more, are used simultaneously for uplink traffic channel segments.Cell 1′ 626 includes (MN 1′″ 1 654, MN N′″ 656) communicating withterrestrial BS 1′ 620 via wireless links (666, 668), respectively. Cell2′ 628 includes (MN1′″ 658, MN N′″ 660) communicating with terrestrialBS 2 622 via wireless links (670, 672), respectively. Cell N′ 630includes (MN1′″″ 662, MN N′″″ 664) communicating with terrestrial BS N′624 via wireless links (674, 676), respectively.

FIG. 6 is a drawing showing exemplary backhaul interconnectivity betweenthe various satellite based and terrestrial based base stations of FIG.5. Various network nodes (702, 704, 706, 708, 710, 712) may, include,e.g., routers, home agent nodes, foreign agent nodes, AAA server nodes,and satellite tracking/high communications data rate capacity groundstations for supporting and communicating with the satellites over thebackhaul network. The links (714, 716, 718) between the network nodes(702, 716, 718) serving as ground stations and the satellite basestations (608, 610, 612) may be wireless links using directed antennaswhile, the links (720, 722, 724, 726, 728, 730, 732, 734, 736, 738)between the terrestrial nodes may be wire and/or wireless links, e.g.,fiber optic cables, broadband cables, microwave links, etc.

FIG. 7A is a drawing 800 illustrating an exemplary satellite 2 604including its exemplary satellite base station 608 and correspondingcellular coverage area (cell 2) 616 on the surface of the earth. MN 1′636 is located near the center of the cell 616 and is closer to thesatellite 604 than is MN N′ 638 which is situated near the outerperimeter of the cell 616. In this example, the beam from the satellitecovers a large geographic area, and there is a significant difference inthe round trip time (RTT) (WT-BS-WT) for the two different MNs, withMN1′ 636 having the shorter RTT. To resolve TRR ambiguity, in accordancewith the present invention, a ranging scheme capable of resolvingdelta-RTT of several milliseconds is implemented.

Typically, in a conventional, mode of operation, there are accessintervals built-in to the system's timing structure where WTs, which maynot be precisely timing synchronized or power controlled, may send arequest signal on an uplink tone, e.g., a contention based uplink tone,to connect and synchronize with a base station and to use that BS as itspoint of network attachment. One exemplary scheme of resolving RTTconsiderations for the satellite based one-tone, in accordance withvarious embodiments, of the present invention, using the accessinterval, e.g., the same access interval used in the conventional modeof operation, with additional time varying coding on the access tone setto indicate which forward link super slot the reverse-link transmissionis associated with. This coding can be used to resolve ambiguity to thesuperslot level. For example, a superslot may be approximately 11.4 msecin duration corresponding to 114 successive OFDM symbol transmissiontime intervals. The wireless terminal may need to try repeated accessattempts at varying time offsets to cover the super-slot (<11.4 msec)ambiguity.

FIG. 8 illustrates a drawing 800 of an exemplary hybrid system includingboth terrestrial and satellite based base stations and a wirelessterminal utilizing terrestrial base station location information toreduce round trip timing ambiguity with respect to a satellite basestation. Exemplary WT (MNA) 902 has been previously connected toterrestrial BS 2′ 622 in cell 2′ 628, but has moved into cell 2 616covered by satellite BS 2. MN A 902 seeks to establish a wireless linkwith the satellite BS 2 608 but needs to resolve timing ambiguity. Inaccordance with a feature of the present invention, the WT includesinformation associating the position of terrestrial base stations withcells of satellite base stations. In some embodiments, multipleterrestrial base stations may be associated with the same satellite cellcoverage area (See FIG. 8A). MNA 902 uses information about the positionof the last terrestrial base station 622 detected to form an initial RTTestimate. In this manner, in accordance with the invention, theambiguity associated with the RTT can be compressed. In some suchembodiments, the ambiguity can be compressed to within the rangesupported by the access protocol used with a terrestrial base station.

FIG. 8A illustrates an exemplary embodiment, in accordance with thepresent invention, where multiple base stations are associated with thesame satellite coverage area. Three exemplary base stations are shownfor the purposes of illustration, although it is understood that ingeneral there may be many more terrestrial base stations within orassociated with a satellite base station's cellular coverage area, as aterrestrial BS may typically have a cellular cover area on the surfaceof the earth with a radius of approximately 1-5 mi while a satellite maytypically have a cellular coverage area on the surface of the earth witha radius of approximately 100-500 mi. Terrestrial base stations (BS A956, BS B 958, BS C 960) with corresponding cells (962, 964, 966) areassociated with the coverage area (cell D 954) corresponding tosatellite D 950, which includes satellite BS D 952. A wireless terminal,which does not know its precise position and is seeking to establish aconnection with satellite D BS 952 can estimate its round trip signaltime based on known position information of the location of terrestrialbase stations, the known position of the satellite base station ingeo-stationary orbit, and signaling information with regard toterrestrial base stations, e.g., using the known position of the lastterrestrial base station to which the WT was connected as a startingpoint. For example, terrestrial BS A 956, which is located near theouter limit of the cell 954 may correspond to an estimated valuerepresenting the longest RTT, terrestrial BS B 958 located at anintermediate point between the outer limit of the cell and the center ofthe cell may represent an intermediate RTT, while terrestrial BS C 960located near the center of the cell 954 may represent the shortest RTT.

FIG. 7 is a flowchart 1200 of an exemplary method of operating awireless terminal, e.g., mobile node, in accordance with the presentinvention. The wireless terminal may be one of a plurality of first typewireless terminals in an exemplary wireless OFDM multiple access spreadspectrum communications system including a plurality of base stations,some base stations being terrestrial based and some base stations beingsatellite based, said first type wireless terminals being capable ofcommunicating with both terrestrial base stations and satellite basestations. The exemplary communications system may also include exemplarysecond type wireless terminals which can communicate with terrestrialbase stations, but cannot communicate with satellite base stations.

Operation of the method of flowchart 1200 starts in step 1202 inresponse to a wireless terminal having powered on or in response to ahandoff operation. Operation proceeds from start step 1202 to step 1204.In step 1204, the wireless terminal determines whether the networkattachment point, that it intends to use as its new point of networkattachment, is a terrestrial base station or a satellite base station.If it is determined in step 1204 that the new network attachment pointis a terrestrial base station then operation proceeds to step 1206,where the wireless terminal sets its operating mode to a first operatingmode, e.g., a multiple tone uplink mode of operation. However, if it isdetermined in step 1204 that the new network attachment point is asatellite base station, then operation proceeds to step 1208, where thewireless terminal sets its operating mode to a second operating mode,e.g., a one tone uplink mode of operation.

Returning to step 1206, operation proceeds from step 1206 to step 1210,where the WT having been accepted by the new terrestrial base station,receives a base station assigned wireless terminal user identifier.Operation proceeds from step 1210 to step 1212, 1214, and 1216. In step1212, the WT is operated to receive signals corresponding to downlinktraffic channel segments, conveying downlink user data, from theterrestrial base station. Operation proceeds from step 1212 to step1218, where the WT sends an Acknowledgment/Negative Acknowledgment(Ack/Nak) response signal to the base station.

Returning to step 1214, in step 1214, the WT determines a dedicatedcontrol channel logical tone from the WT user ID received in step 1212.Operation proceeds from step 1214 to step 1220. In step 1220, the WTdetermines the physical tone corresponding to the logical tone to usebased upon tone hopping information. For example, the WT assigned IDvariable may have a range of 32 values (0 . . . 31), each IDcorresponding to a different single logical tone in a uplink timingstructure, e.g., an uplink timing structure including 113 tones. The 113logical tones may be hopped in accordance with an uplink tone hoppingpattern within the uplink timing structure. For example, excludingaccess intervals, the uplink timing structure may be subdivided intodwell intervals, each dwell interval having a duration of a fixednumber, e.g., seven, successive OFDM symbol transmission time intervals,and tone hopping occurs at the dwell boundaries but not in-between.Operation proceeds from step 1220 to step 1222. In step 1222, the WT isoperated to transmit uplink control channel signals using the dedicatedcontrol channel tone.

Returning to step 1216, in step 1216, the WT checks as to whether thereis user data to transmit on the uplink. If there is no data waiting tobe transmitted, operation proceeds back to step 1216, where the WTcontinues to check for data to transmit. However, if in step 1216, it isdetermined that there is user data to transmit on the uplink, thenoperation proceeds from step 1216 to step 1224. In step 1224, the WTrequests an uplink traffic channel assignment from the terrestrial basestation. Operation proceeds from step 1224 to step 1226. In step 1226,the WT receives an uplink traffic channel segment assignment. Operationproceeds to step 1228, where the WT selects a modulation method to use,e.g., QPSK or QAM16. In step 1230, the WT selects a coding rate to beused. Operation proceeds from step 1230 to step 1232, where the WT codesthe user data for the assigned uplink traffic channel segment inaccordance with the selected coding rate of step 1230 and maps the codedbits to modulation symbol values in accordance with the selectedmodulation method of step 1228. Operation proceeds from step 1232 tostep 1234, where the WT determines the logical tones to use based on theuplink traffic channel segment assignment. In step 1236, the WTdetermines the physical tones, corresponding to the logical tones to usebased on tone hopping information. Operation proceeds from step 1236 tostep 1238. In step 1238, the WT transmits user data to the terrestrialbase station using the determined physical tones.

Returning to step 1208, operation proceeds from step 1208 to step 1240.In step 1240, the WT, having been accepted by the satellite basestation, receives a BS assigned WT user ID from the satellite basestation. Operation proceeds from step 1240 to steps 1242 and step 1244.

In step 1242, the WT is operated to receive signals corresponding todownlink traffic channel segments, conveying downlink user data, fromthe satellite base station. Operation proceeds from step 1242 to step1246, where the WT request retransmission of the downlink trafficchannel user data in response to an error. If the downlink transmissionwas successfully received and decoded no response is communicated fromthe wireless terminal to the base station. In some embodiments, where anerror is detected in the information recovery process, a request forretransmission is not sent, e.g., as the time window of validity for thelost downlink data will expire before a retransmission could becompleted or due to a low priority level of the data.

Returning to step 1244, in step 1244, the WT determines the singleuplink logical tone to use for both control data and user data for theassigned WT user ID. Operation proceeds to either step 1248 or step1250, depending on the particular embodiment.

In step 1248, the WT multiplexes user data and control data to becommunicated on the uplink. The multiplexed data of step 1248 isforwarded to step 1252, where the WT codes the mixture of user andcontrol information bits into a single coded block. Operation proceedsfrom step 1252 to step 1254, where the WT determines the physical toneto use for each dwell based on the determined logical tone and tonehopping information. Operation proceeds from step 1254 to step 1256. Instep 1256, the WT is operated to transmit the coded block of combineduser data and control data to the satellite base station using thedetermined physical tone for each dwell.

In step 1250, the WT is operated to code the user data and control datain independent blocks. Operation proceeds from step 1250 to step 1258,where the WT is operated to determine the physical tone to be used foreach dwell based on the determined logical tone and the tone hoppinginformation. Operation proceeds from step 1258 to step 1260. In step1260, the WT is operated to transmit coded blocks of user data and codedblocks of control data to the satellite base station using thedetermined physical tone, determined on a per dwell basis. With regardto step 1260, in accordance with a feature of some embodiments of thepresent invention, during time intervals dedicated to user data, wherethere is no user data to be transmitted, the single tone is allowed togo unused.

Operating a wireless terminal in accordance with the method of flowchart1200 can result in operating the wireless terminal during a first periodof time including a first plurality of consecutive OFDM symboltransmission time periods in the first mode of operation during whichmultiple OFDM tones are used simultaneously to transmit at least someuser data in a first uplink signal having a first peak to average powerratio. For example, the WT may using a terrestrial base station as itspoint of network attachment and may be communicating uplink user dataover air link resources corresponding to an uplink traffic channelsegment using a plurality of tones simultaneously for uplink trafficchannel data, e.g., 7, 14, or 28 tones; an additional tone or tones mayalso be used in parallel for control signaling, e.g., a dedicatedcontrol channel tone. Operating a wireless terminal in accordance withthe method of flowchart 1200 can also result in operating the wirelessterminal during a second period of time including a second plurality ofconsecutive OFDM symbol transmission time periods in the second mode ofoperation during which at most one OFDM tone is used to transmit atleast some user data in a second uplink signal having a second peak toaverage power ratio, which is different from said first peak to averageratio. For example, during the second period of time, the WT may beusing a satellite base station as its point of network attachment andmay be communicating uplink user data and control data over air linkresources corresponding to a single dedicated logical tone associatedwith a base station assigned WT user identifier, said single dedicatedlogical tone may be hopped to different physical tones on dwellboundaries.

In some embodiments, the second peak to average power ratio is lowerthan the first peak to average power ratio, e.g., by at least 4 dB. Insome embodiments, the WT uses an omni-directional antenna. User datacommunicated over the uplink during the first mode of operation duringthe first period of time can include user data at a rate of at least 4.8Kbits/sec. User data communicated over the uplink during the second modeof operation during the second period of time can include user data at arate of at least 4.8 Kbits/sec. For example, a voice channel may besupported for WT operation in both the first and second modes ofoperation. In some embodiments, the WT supports a plurality of differentuplink coding rate options in the first mode of operation including aplurality of different coding rates and a plurality of differentmodulation schemes, e.g., QPSK, QAM16. In some embodiments, the WTsupports a single uplink rate option for operation in the second mode,e.g. QPSK using a single coding rate. In some embodiments, theinformation bit rate, regarding uplink user data signals, in the secondmode of operation is less than or equal to the minimum information bitrate, regarding uplink user data signal, in the first mode of operation.

In some embodiments, the distance between the satellite base station andthe wireless terminal, when said satellite base station is being used bythe WT as its point of network attachment, is at least 3 times thedistance between the terrestrial base station and the wireless terminal,when said terrestrial base station is being used by the WT as its pointof network attachment. In some embodiments, at least some of thesatellite base stations in the communications system are geo-stationaryor geo-synchronous satellites. In some such embodiments, the distancebetween the geo-stationary or geo-synchronous satellite base station andthe WT using it as its point of network attachment is at least 35,000km, while the distance between a ground base station and the WT using itas its point of network attachment is at most 100 km. In someembodiments, the satellite base station being used by the WT as itspoint of network attachment is at least a distance away from the WT suchthat a signal round trip time exceeds 100 OFDM symbol transmission timeperiod, each OFDM symbol transmission time period including an amount oftime used to transmit one OFDM symbol and a corresponding cyclic prefix.

In some embodiments, switching from a first mode of operation to asecond mode of operation occurs when a handoff occurs between aterrestrial base station and a satellite base station. In some suchembodiments, wherein switching from the first mode of operation to thesecond mode of operation occurs, the WT ceases to send acknowledgmentsignals in response to received downlink user data. In some suchembodiments, wherein switching from the first mode of operation to thesecond mode of operation occurs, the WT reduces the frequency and/ornumber of uplink control signals which are transmitted.

Other embodiments, in accordance with various features of the presentinvention, may include systems that include space based base stationsbut do not include terrestrial based base stations, systems that includeterrestrial base stations but do not include space based base stations,and various combinations including airborne platform based basestations.

In various embodiments of the invention when communicating with remotebase stations, some of which use multiple tones in an uplink, uplinksegment assignments are used with the UL assignment slave structurebeing adjusted to account for assignment of traffic segments>2.times.the maximum RTT (round trip time). In some but not necessarily all casesof terminals without high gain antennas, e.g., handsets withomni-directional antennas or nearly omni-directional antennas, theextreme link budget requirements for successful receipt of a transmittedsignal by a satellite base station may limit communication through theuse of single one mode. Accordingly, in some embodiments when a handoffoccurs from a terrestrial base station to a satellite base station, thewireless terminal detects the change and switches from multi-tone uplinkmode to a single OFDM tone uplink mode operation.

For geo-stationary satellites with a beam covering a large geographicalarea there may be a significant difference in the round trip timebetween the center of the beam and the edge. To resolve this RTTambiguity a ranging scheme capable of resolving delta-RTT of severalmilliseconds may be desirable.

Such a scheme can use the existing access interval in OFDM withadditional time varying coding on the access tone set to indicate whichforward link super slot the revere-link transmission is associated with.This coding can resolve ambiguity to the super slot level. The terminalmay need to try repeated access attempts at varying time offsets tocover the sub-superslot (<11.4 msec) ambiguity. For a hybridterrestrial-satellite network the terminal can use information about theposition of the last terrestrial base station detected to form aninitial RTT estimate and compress the ambiguity to within the rangesupported by the normal access protocol.

FIG. 9 is a drawing 1000 illustrating that round trip signal delaybetween a satellite base station and a terrestrial located WT will begreater than a superslot. Drawing 1000 includes a horizontal axis 1002representing time, an access probe signal 1004 being sent from aterrestrially located wireless terminal to the satellite base station,and a response signal 1006 from the satellite base station beingreceived by the terrestrially located wireless terminal. Round tripdelay time 1008 is greater than a super-slot time interval. For example,in some terrestrial wireless communications systems, an access intervalis structured once every superslot providing an opportunity for awireless terminal to request to establish a connection with a newterrestrial BS and timing synchronize. In the case of a terrestriallylocated wireless terminal seeking access with a terrestrial basestation, where the round trip distance is relatively short, e.g.,typically 2-10 miles, the round trip signal travel time is approximately11 micro-sec to 54 micro-sec, and the round trip delay including signalprocessing by the terrestrial base station can be within a super-slot,e.g., a time interval of 114 super-slots representing approximately 11.4msec. Therefore, there is no ambiguity with respect to which superslotsthe access probe and response signal are associated with. On the otherhand, in the case of a terrestrial wireless terminal seeking access witha satellite base station in geo-synchronous orbit of approximately22,300 mi with a round trip signal travel time is approximately 240msec, the round trip delay will be greater than a super-slot intervaltime of 11.4 msec. In addition, there can be variation in the round tripdelay due to the large coverage area of the satellite base stationresulting in different RTTs depending upon the location of the WT withinthe cell. In accordance, with the present invention, the access methodof a WT seeking to establish a wireless link with the satellite BS andtiming synchronize is modified to address timing ambiguity issues thatare present when a WT seeks to connect to a satellite BS which are notpresent when the WT seeks to connect to a terrestrial BS.

FIG. 10 is a drawing 1100 illustrating one feature of the presentinvention used in the access process to determine timing synchronizationbetween the satellite base station and the WT. FIG. 10 illustrates thatthe exemplary timing structure is sub-divided into superslots, e.g., 114OFDM symbol time intervals, with the start of each superslot being anaccess interval, e.g., 9 OFDM symbol time intervals. Drawing 1100includes a horizontal axis 1102 representing time, superslot 1 1104,superslot 2 1106, superslot N 1108. Superslot 1 1104 includes exemplaryterrestrial access interval 1110; superslot 2 1106 includes exemplaryterrestrial access interval 1112; superslot N includes exemplaryterrestrial access interval 1114. The base station can send out areference signal, e.g., a beacon signal, defining a beacon slot, and thesuperslots can be indexed within the beacons slot. With the terrestrialBS, the WT that seeks to establish a link with a BS sends access probesignal during the access interval and the BS receiving the signal, cansend back a WT identifier and a timing correction to providesynchronization. However, in the case of the satellite BS, the timingambiguity is greater than a superslot. Therefore, the WT can code theaccess signal probe differently depending upon which superslot it wassent from. Coded access probe signal 1116, which occurs within accessinterval 1110, is coded to identify superslot 1 1104. Coded access probesignal 1118, which occurs during access interval 1112, is coded toidentify superslot 2 1106. Coded access probe signal which occurs duringaccess interval 1114 is coded to identify superslot N 1108. Therefore,when the base station receives the coded access probe signal, the BS candetermine from the code, the superslot it was sent from.

FIG. 11 is a drawing 1200 illustrating another feature of the presentinvention used in the access process to determine timing synchronizationbetween the satellite base station and the WT. FIG. 11 illustrates thatfrom the WTs perspective, the WT can offset the access probe signal,e.g., by different offsets, e.g., a 400 micro-second offset, such thatthe satellite can further resolve timing synchronization to within thesuperslot. Drawing 1200 includes a horizontal axis 1150 representingtime, superslot 1 1152, superslot 2 1154, and superslot N 1156.Superslots (1152, 1154, 1156) include time intervals (1158, 1160, 1162),e.g., 9 OFDM symbol transmission time intervals at the start of eachsuperslot, typically used for providing an opportunity for a WT to sendan access probe signal to a terrestrial base station to establish aconnection and timing synchronize. When operating in a mode to attemptaccess with a satellite base station, the WT can send access probes atdifferent times, e.g., including times outside intervals (1158, 1160,1162), within a superslot with respect to the WT's reference. Multipleaccess probe signals (1164, 1166, 1168, 1170, 1172, 1174, 1176) areshown with exemplary spacing offset between access probe signals being400 micro-seconds, illustrating that access probes may occur at varioustimes within a superslot. Access probe signals sent during superslot 11152, e.g., access probe signal (1164, 1166, 1168, 1170, or 1172) arecoded to identify superslot 1. Access probe signals sent duringsuperslot 2 1154, such as access probe 1174 are coded to identifysuperslot 2. Access probe signals sent during superslot N 1156, such asaccess probe signal 1176 are coded to identify superslot N.

The terrestrial located WT which is not tightly synchronized to thesatellite base station, and in which there is a large degree ofuncertainty in the timing due to large possible distance variationsbetween the satellite and the WT, can monitor for access probe signalsfrom WTs for a short interval within a superslot, e.g., the sameinterval corresponding to that used by a terrestrial base station. Ifthe transmitted WT probe signal does not hit the access interval windowof opportunity for reception in the satellite base station, thesatellite base station will not decode the request. The WT, by sendingmultiple requests with different offsets can span the potentialvariation in timing, and eventually, a WT probe signal should becaptured and decoded by the satellite BS. Then, the satellite BS, bydecoding the signal can identify the superslot from which the signal wasdirected and resolve the timing to within the superslot, and thesatellite BS can send a BS assigned WT identifier and a timingcorrection signal to the WT. The WT can apply the received timingcorrection information to synchronize with the satellite base station.

FIG. 12 further illustrates the concept of the WT sending multipleaccess probes to the satellite base station with different timingoffsets. FIG. 12 is a drawing 1169 including a horizontal axis 1171representing time which shows ranges during which the WT sends accessprobes to the satellite base station. FIG. 12 includes: a firstsuperslot used by the WT for sending an access probe signal 1175 duringwhich the WT sends coded access probe signal 1177 in accordance with afirst timing offset value t.sub.0 1179, a second superslot used by theWT for sending an access probe signal 1180 during which the WT sendscoded access probe signal 1182 in accordance with a second timing offsett.sub.0+DELTA 1184, and an Nth superslot used by the WT for sending anaccess probe signal 1186 during which the WT sends coded access probesignal 1188 in accordance with an Nth timing offset value t.sub.0+NDELTA1190. Consider that the satellite BS will accept the one of the accessprobes, e.g., the kth probe, which happens to fall within the accessinterval monitored by the BS for accepting and processing access probesignals from WTs.

For example, consider that the ambiguity in timing between the satelliteBS and the terrestrial WT is greater than a superslot. The WT seeks toconnect to the satellite BS. The satellite BS is outputting beaconsignals, each beacon signal associated with a beacon slot and a set ofsuperslot. Each superslot has an access interval, e.g., 9 OFDM symbolsduring which the BS accepts coded access probes from WTs seeking toestablish a connection with the satellite BS. If the access probe isoutside this access interval window, from the perspective of the BSreceiving the signal, the BS will not accept the signal. The WT seekingto use the satellite BS as its point of network attachment sends a codedaccess probe signal, coded to signify the super-slot index number.Since, the WTs access probe may be outside the window of acceptance whenit reaches the BS, the WT may send out multiple probes, with differenttiming offsets, e.g., with respect to the start of a superslot. Forexample a timing offset of 400 micro-sec may be used. For example, a WTmay send out a sequence of access probes, e.g., 10 access probes, atintervals of approximately ½ sec apart, with each successive accessprobe having a different timing offset with respect to the start of asuperslot. However, the BS will only recognize the access probe signalwhich is received within its access interval window. Access probesignals outside the window are tolerated by the system as interferencenoise. When, the BS receives the one of the multiple access probes fromthe WT which is received within the access interval window, the BSdetermines the superslot information by decoding the signal, anddetermines a timing correction for achieving timing synchronizationbetween the BS and WT. The BS sends a base station assigned WTidentifier, a repeat of the superslot identification information, and atiming correction value to the WT. The WT can receive the base stationassigned WT identifier, apply timing correction, and thus is allowed touse the satellite BS as its point of network attachment. A singlededicated logical uplink tone may be associated with the assigned WTidentifier for the WT to use for uplink signaling to the satellite BS.

FIG. 13 is a drawing 1300 illustrating exemplary access signaling inaccordance with methods of the present invention. FIG. 13 includes anexemplary base station 1302 and an exemplary wireless terminal 1304,implemented in accordance with the present invention. Exemplary BS 1302transmits downlink signaling using a downlink timing and frequencystructure. The downlink timing structure includes beacons slots, eachbeacon slot including a fixed number of indexed superslots, e.g. 8indexed superslots per beacon slot, and, each superslot including afixed number of OFDM symbol transmission time intervals, e.g., 114 OFDMsymbol transmission time intervals per superslot. Each beacon slot alsoincludes a beacon signal. Downlink signals from BS 1302 are received byWT 1304, the downlink signaling delay between when the BS 1302 transmitsand when the WT 1304 receives varies as a function of the distancebetween the BS and WT. Received beacon signal 1306 is shown with thecorresponding beacon slot 1308 including indexed superslots (superslot 11310, superslot 2 1312, superslot 3 1314, . . . , superslot N 1316). WT1304 can reference uplink signaling with respect to the receivedbeaconslot timing.

The BS 1302 also maintains an uplink timing and frequency structuresynchronized at the base station with respect to the downlink timingstructure. Within the uplink timing and frequency structure at BS 1302,there are receive windows for receiving access signals, e.g., one windowcorresponding to each superslot (1318, 1320, 1322, . . . , 1324).

WT 1304 sends an uplink access probe signal 1326 to BS 1302 seeking togain access and register with BS 1302. Arrows (1328, 1330, 1332)indicates cases (A, B, C) of (shorter, intermediate, and longer)propagation delays corresponding to (short, intermediate, and long)distances, respectively, between BS 1302 and WT 1304.

In exemplary case A, the WT 1304 has sent access probe signal 1326 andit has successfully hit access window of opportunity 1318. BS 1302 canprocess the access probe signal, determine a timing offset and send thetiming offset correction to WT 1304, allowing the WT 1304, to use thereceived timing offset correction to adjust uplink transmission timingto more precisely timing synchronize its uplink signaling, such that theuplink signals from WT 1304 arrive synchronized with BS 1302 uplinkreceive timing, e.g., allowing data communications.

In exemplary case B, the WT 1304 has sent access probe signal 1326 andit has missed the access windows of opportunity (1318, 1320). BS 1302does not successfully process the access probe signal, the access probesignal is treated by BS 1302 as interference, and BS 1302 does notrespond to WT 1304.

In exemplary case C, the WT 1304 has sent access probe signal 1326 andit has successfully hit access window of opportunity 1320. BS 1302 canprocess the access probe signal, determine a timing offset correctionand send the timing offset correction to WT 1304, allowing the WT 1304,to use the received timing offset to adjust uplink transmission timingto more precisely timing synchronize its uplink signaling, such that theuplink signals from WT 1304 arrive synchronized with BS 1302 uplinkreceive timing, e.g., allowing data communications.

In some embodiments, e.g., with nearby terrestrial base stations such asa terrestrial BS with a cell radius of 5 miles, the amount of round triptime uncertainty is relatively small, and the WT 1304 when transmittingan access probe uplink signal can be expect to hit the next accesswindow at the base station. In some embodiments, where the base stationis far away from the WT, but the relative distance uncertainty is verysmall, the access probe signal can be expected to hit an access windowat the base station.

However, in embodiments, where the uncertainty in round trip time islarger than supported by the access interval size, the access probesignal may or may not hit an access window of opportunity. In such acase, if an access probe misses, as in case B above, WT timing needs tobe adjusted and another access probe sent. Access interval window timerepresents signaling overhead and it is desirable to keep the accessinterval short. For example, an exemplary access window time interval is9 OFDM symbol transmission time intervals corresponding to an exemplarysuperslot of 114 OFDM symbol transmission time intervals.

In the examples of FIG. 13, it should be observed that the variation inpropagation delay can be such that the access probe signal 1326 couldhit different access windows 1318, 1320, e.g., depending upon therelative distance between WT 1304 and BS 1302. For example, considerthat case A (arrow 1328) and case C (arrow 1332) correspond to the sameBS whose relative distance to WT can vary to an extent that an accessprobe signal, when successfully received, may be received in differentones of access windows depending upon the relative BS-WT distance at agiven time. Also consider that the WT is allowed to transmit accessprobe signals during superslots having different index values. When theBS receives an access probe signal, for the BS to calculate the correcttiming correction, the base station needs to know more information fromthe WT 1304 in order to gain a timing reference point. In accordancewith one feature of some embodiments of the present invention, the WTcodes the access probe signal 1326 to identify the superslot index fromwhich access probe signal 1326 was transmitted. The BS 1302 uses theslot index information to calculate a timing offset correction, which issent via a downlink signal to WT 1304. WT 1304 receives the timingcorrection signal and adjusts its uplink timing accordingly.

In some embodiments of the present invention, an alternative method isemployed, wherein the access probe signal does not code the superslotindex; however, the base station communicates via the downlink a timingcorrection signal and a slot index offset indicator, e.g.,distinguishing between access window 1318 and access window 1320. Then,the WT 1304, which knows the superslot index of the transmitted accessprobe signal can combine the information with the received timingcorrection signal and the received slot index indicator to calculate acomposite timing adjustment, and apply the timing adjustment.

FIG. 14 is a drawing 1400 illustrating exemplary access signaling inaccordance with methods of the present invention. FIG. 14 includes anexemplary base station 1402 and an exemplary wireless terminal 1404,implemented in accordance with the present invention. Exemplary BS 1402transmits downlink signaling using a downlink timing and frequencystructure. The downlink timing structure includes beacons slots, eachbeacon slot including a fixed number of indexed superslots, e.g. 8indexed superslots per beacon slot, and, each superslot including afixed number of OFDM symbol transmission time intervals, e.g., 114 OFDMsymbol transmission time intervals per superslot. Each beacon slot alsoincludes a beacon signal. Downlink signals from BS 1402 are received byWT 1404, the downlink signaling delay between when the BS 1402 transmitsand when the WT 1402 receives varies as a function of the distancebetween the BS and WT. Received beacon signal 1406 is shown with thecorresponding beacon slot 1408 including indexed superslots (superslot 11410, superslot 2 1412, superslot 3 1414, . . . , superslot N 1416). WT1404 can reference uplink signaling with respect to the receivedbeaconslot timing. The RTT uncertainty is such that the WT 1404, whensending an access probe signal may or may not be successful in hittingan access slot at the base station 1402.

The BS 1402 also maintains an uplink timing and frequency structuresynchronized at the base station with respect to its downlink timingstructure. Within the uplink timing and frequency structure at BS 1402,there are receive windows for receiving access signals, access slots,e.g., one window corresponding to each superslot (1418, 1420, 1422, . .. , 1424). In addition, the uplink timing is structured such that thereare data slots (1426, 1428, 1428) between the access slots.

FIG. 14 illustrates a method, in accordance with the present invention,of adjusting access probe timing offsets with respect to the start of asuperslot, such that an access probe uplink signal will be eventuallyreceived within an access slot. This method is useful in cases wherevariation in signal RTT, e.g., due to potential variations in BS-WTdistance, is such that hitting an access window on the first attempt isnot ensured.

WT 1404 transmits access probe signal 1432, the transmission timingbeing controlled such that there is a first timing offset, timing offsett.sub. 1 1434 with respect to the start of the superslot during whichthe signal is transmitted. The transmitted access probe signal 1432 isan uplink signal which is delayed by signaling propagation asrepresented by slanted arrow 1433 and arrives as access probe signal1432′ at the receiver of BS 1402. However, access probe signal 1432′happens to arrive during data slot 1426, and thus is considered to beinterference by BS 1402. BS 1402 does not send a response to WT 1404.

WT 1404 adjusts its timing offset to a 2.sup.nd timing offset valuet.sub.2 1438 and transmits access probe signal 1436. The transmittedaccess probe signal 1436 is an uplink signal which is delayed bysignaling propagation as represented by slanted arrow 1437 and arrivesas access probe signal 1436′ at the receiver of BS 1402. However, thistime the received access probe signal 1436′ is within access slot 1420,and the BS 1402 processes the access signal, accepts WT 1404 to beregistered, calculates a timing correction signal and sends the timingcorrection signal via the downlink to WT 1404. The WT adjusts it uplinktiming in accordance with the received timing correction signal.

Differences between access probe signaling timing offsets can be chosenin correlation to the size of access slot such that successive accessprobes with different offsets will eventually hit an access slot. Forexample in an exemplary system with access slots of 9 OFDM symboltransmission time intervals, different time offsets may differ by 4 OFDMsymbol transmission time intervals, e.g., with an OFDM transmission timeinterval being approximately 100 micro-sec.

FIG. 15 is a drawing 1500 illustrating exemplary access signaling inaccordance with methods of the present invention. FIG. 15 includes anexemplary base station 1502 and an exemplary wireless terminal 1504,implemented in accordance with the present invention. Consider that theexemplary BS 1502 may be a satellite BS in geo-stationary orbit having alarge cellular coverage area on the surface of the earth, e.g., with aradius of 100, 200, 500 or more miles. In such as embodiment, considerthat the RTT is greater than a superslot in the downlink, and that theRTT uncertainty, e.g., due to potential WT 1504 location variation, suchthat an exemplary access probe signal may or may not hit access timeslot at the base station 1500. In this exemplary embodiment, the twofeatures described above, coding superslot index identificationinformation into the access probe and sending successive access probeswith different timing offsets from the start of the superslot in whichthe access signal is transmitted, are used in combination to obtain atiming correction for the WT 1504.

BS 1502 transmits downlink signals including a downlink beacon signalper beaconslot which is part of a downlink timing structure includingsuperslots, the downlink timing structure known to the BS and WT. The WT1504 is able to synchronize with respect to the received downlinksignals and can identify the index values of superslots within eachbeacon slot.

WT 1504 decides that it would like to use BS 1502, a satellite BS, as apoint of network attachment; however, WT 1504 does not know its positionand thus does not know the RTT. WT 1504 sends access probe signal 1508with a 1.sup.st timing offset t.sub.1 1510, with respect to the start ofthe superslot 1512 during which the signal is transmitted. The indexnumber of superslot 1512 within its beaconslot is known to WT 1504 andencoded in the access probe signal 1508. After a WT-BS propagation delaytime, the access signal arrives at BS 1502 as access probe 1508′.However, the access probe signal 1508′ hits data slot 1514, rather thanan access slot. BS 1502 treats signal 1508′ as interference and does notrespond to WT 1504.

Wireless terminal 1504 waits for time interval 1516 before sendinganother access probe signal. Time interval 1516 is chosen to be greaterthan the RTT plus some additional time allowed for signal processing,providing enough time for a BS 1502 access probe response signal to begenerated, transmitted, propagate, and be detected by WT 1504, if theaccess probe signal had successfully hit an access slot at BS 1502 andBS 1502 had accepted WT 1504 for registration.

Having not received a response in the expected time interval, WT 1504adjusts its timing offset from the start of a superslot to a 2.sup.ndtiming offset 1518, different than the first timing offset 1510, andsends another access probe signal 1520 during superslot 1522. The indexnumber of the superslot 1522 within its beacon slot is coded in signal1520, the index value may be the same or different than the index valuecoded in signal 1508. After a WT-BS propagation delay time, the accesssignal arrives at BS 1502 as access probe 1520′. In this case, theaccess probe signal 1520′ hits access slot 1521. BS 1502 decodes thesuperslot index communicated, measures a received signal 1520′ timingoffset within the access slot 1521, and uses the measured timing offsetand the superslot information to calculate a timing correction value forthe WT 1504. BS 1502 sends the timing offset correction value as adownlink signal to WT 1504. WT 1504 receives and decodes the timingoffset value and adjusts its uplink timing in accordance with thereceived correction. WT 1504 received signaling identifying that it isbeing accepted for registration by BS 1502, before the time that the WT1504 would attempt to transmit another access probe signal, e.g., with adifferent offset.

FIG. 16 is a flowchart 1600 of an exemplary method of operating awireless terminal to access a base station and perform a timingsynchronization operation in accordance with the present invention.Operation starts in start step 1602, where the WT is powered on,initialized, and starts to receive downlink signals from one or morebase stations. Operation proceeds from step 1602 to step 1604.

In step 1604, the WT decides as it whether it is seeking to initiateaccess with a satellite or terrestrial base station. The exemplary WT,implemented in accordance with the present invention, may includeimplementation of different methods of access. A first method of accessis tailored to satellite base stations, e.g., satellite base stations ingeo-stationary orbit with cell coverage areas on the surface of theearth having a radius of approximately 100-500 mi, where the signal RTTis greater than a superslot, and the ambiguity in RTT is greater thanthe access time interval. A second method of access is tailored toterrestrial base stations, e.g., with a relatively small cell radius,e.g., 1, 2, or 5 mi, where the signal RTT is less than a superslot, andthe ambiguity in RTT is small enough such that an access request signaltransmitted from the WT should be expected to hit an access slot at theterrestrial BS on a single attempt. If the WT is seeking access with asatellite BS, operation proceeds from step 1604 to step 1606; while ifthe WT is seeking to access a terrestrial base station, operationproceeds from step 1604 to step 1608.

In step 1606, the WT is operated to receive a downlink beacon signal orsignals from a satellite BS. The downlink timing and frequency structureused by the satellite base station in the exemplary system may includebeacon slots which occur on a recurring basis, with each beaconslotincluding a beacon signal and with each beaconslot including a fixednumber of superslots, e.g., eight, each of the superslots within abeaconslot being associated with an index value, and each of thesuperslots including a fixed number of OFDM symbol transmission timeintervals, e.g., 114.

Operation proceeds from step 1606 to step 1608. In step 1608, the WTdetermines from the received beacon signal(s) a timing reference, e.g.,determining the start of a beaconslot with respect to the receiveddownlink signaling. In step 1610, the WT sets a probe counter equal to1, and in step 1612 the WT sets a timing offset variable equal to aninitial timing offset; e.g., the initial timing offset being apredetermined value stored in the WT. Operation proceeds from step 1612to step 1614.

In step 1614, the WT selects a superslot within a beaconslot fortransmitting a first access probe signal and identifies the index of theselected superslot. Then, in step 1614, the WT codes the index of theselected superslot into the first access probe signal. Next, in step1618, the WT transmits the first access probe signal at a point in timeoccurring within the selected superslot such that the transmission istiming offset from the start of the selected superslot by the timingoffset value of step 1612. Operation proceeds from step 1618 viaconnecting node A 1620 to step 1622.

In step 1622, the WT is operated to receive downlink signaling from thesatellite base station, the received downlink signaling may include aresponse to the access probe signal. Operation proceeds from step 1622to step 1624. In step 1624, the WT checks as to whether a response wasreceived directed to the WT. If a response was not received, operationproceeds from step 1624 to step 1626; however if a response was receiveddirected to the WT operation proceeds to step 1628.

In step 1626, the WT checks as to whether the change in time since thelast access probe transmission has exceeded the expected worst caseRTT+processing time, e.g., a predetermined limit value stored in the WT.If the time limit has not been exceeded, then operation returns to step1622, where the WT continues the process of receiving downlink signalsand checking for a response. However, if in step 1626, the WT determinesthat the time limit has been exceeded, then the WT operation proceeds tostep 1630, where the WT increments the probe counter.

Next, in step 1632, the WT checks as it whether the probe counterexceeds a max probe counter number. The max probe counter number may bea predetermined value stored in WT memory selected such that a set ofmax probe counter number access probes with different timing offsetsshould be sufficient to cover the timing ambiguity such that at leastone of the access probes should be expected to be timed to hit an accessslot at the satellite base station.

If the probe counter has exceeded the max probe number in step 1632, itcan be assumed that access attempt set has resulted in failure andoperation proceeds via connecting node B 1634 to step 1604. For example,possible causes of failure may include: interference conditions suchthat the access probe signal that should have hit an access slot at thebase station was not able to be successfully detected and processed, thesatellite BS decided to deny the WT access, e.g., due to loadingconsiderations, or the response signal from the satellite base stationwas not able to be successfully recovered. In step 1604, the WT candecide whether to repeat the process with the same satellite basestation or attempt to access a different base station.

If in step 1632, the probe counter did not exceed the max probe counternumber operation proceeds to step 1636, where the WT sets the timingoffset equal to the current timing offset value plus a delta offset. Forexample, the delta offset can be fraction, e.g., less than half, of theaccess slot interval. Then, in step 1640, the WT selects a superslotwithin a beaconslot for transmitting another access probe signal andidentifies the index of the selected superslot. Next in step 1642, theWT codes the index of the selected superslot into another access probesignal. Then, in step 1644, the WT transmits the another access probesignal at a point in time occurring within the selected superslot suchthat the transmission is timing offset from the start of the selectedsuperslot by the timing offset value of step 1638. Operation proceedsfrom step 1644 via connecting node A 1620 back to step 1622 where the WTreceives downlink signals and checks for a response to the access probesignal.

Returning to step 1624, if in step 1624 it was determined that the WThas received a response directed to the wireless terminal, operationproceeds to step 1628, where the WT processes the received response,directed to the WT including the timing correction information.Operation proceeds from step 1628 to step 1646. In step 1646, the WTadjusts WT timing in accordance with received timing correctioninformation.

Returning to step 1604, in step 1604 if the wireless terminal seeks toinitiate access via a terrestrial BS station, operation proceeds to step1608, where the WT is operated to receive a downlink beacon signal orsignals from the terrrestrial base station, that the WT wishes to use asit point of network attachment. Then, in step 1646, the WT determinesfrom the received beacon signal or signals, a timing reference, and instep 1648, the WT uses the determined time reference to determine whento transmit an access request signal such that the access request signalshould be expected to be received at the terrestrial base station duringan access interval. Operation proceeds from step 1648 to step 1650.

In step 1650, the WT is operated to transmit an access request signalsuch that the access request signal at the determined time, said accessrequest signal not including coded superslot identification information.Next, in step 1652, the WT is operated to receive downlink signalingfrom the terrestrial BS which may include access grant information.Operation proceeds from step 1652 to step 1654.

In step 1654, the WT is operated to determine whether the WT received anaccess grant signal in response to its access request transmission. Ifthe access grant was not received, operation proceeds from step 1654 viaconnecting node B 1634, where the WT decides whether to retry accesswith the same terrestrial base station or to attempt access with adifferent BS. If it is determined in step 1654, that the WT was grantedaccess to use the terrestrial BS as its point of network attachment,then operation proceeds to step 1656, where the WT is operated toprocess the access grant signaling directed to the WT, including timingcorrection information. Then, in step 1658, the WT is operated to adjustWT timing in accordance with the received timing correction informationof step 1656.

FIG. 17 comprising the combination of FIG. 17A and FIG. 17B is aflowchart 1700 of an exemplary method of operating a communicationsdevice for use in a communications system. For the example, theexemplary communications device may be a wireless terminal such as amobile node, implemented in accordance with the present invention, andthe exemplary communications system may be a multiple access spreadspectrum OFDM wireless communications system. The communications systemmay include one or more base stations, and each base station maytransmit downlink beacon signals. The various base stations in thesystem may or may not be timing synchronized with respect to oneanother. In the exemplary communications system, beacon signalingbroadcast by a base station may be used in providing timing referenceinformation with respect to the base station. In the exemplarycommunications system, the timing structure for a base station is suchthat beacon time slots occur on a periodic basis, a beacon signal beingtransmitted by a base station during each beacon time slot according toa periodic downlink timing structure, said downlink timing structureincluding a plurality of superslots within each beaconslot, theindividual superslots within each beacon slot being suitable foridentification through the use of a superslot index, each superslotincluding a plurality of symbol transmission time periods.

Operation starts in start step 1702, where the communications device ispowered on and initialized. Operation proceeds from step 1702 to step1704. In step 1704, the communications device receives at least onebeacon signal from the base station that the communications devicewishes to use a network attachment point, e.g., a satellite BS. In someembodiments the communications device receives multiple beacon signalsand/or other downlink broadcast information from the base station, e.g.,pilot signals, before proceeding. Operation proceeds from step 1704 tostep 1706. In step 1706, the communications device processes thereceived beacon signal to determine a downlink timing reference point,superslots occurring within a beaconslot having a predeterminedreference to the determined timing reference point. Operation proceedsfrom step 1706 to step 1708.

In step 1708, the communications device determines a time at which totransmit a first access probe as a function of the determined timingreference point. For example, the first access probe has an initial timeoffset from the determined timing reference point. In some embodiments,e.g., some hybrid system including both satellite and terrestrial basestations, the communications device performs sub-step 1709, and insub-step 1709, the communications device determines a time at which totransmit a first access probe as a function of location informationdetermined from a signal from a terrestrial base station. In some suchembodiments, determining the time at which to transmit the first accessprobe is further performed as a function of known information indicatingthe location of said terrestrial base station and the location of saidsatellite base station. For example, the base station to which thecommunications device now wishes to send an access probe signal may be asatellite base station, and there may be a relatively large amount ofuncertainty in the timing to use for transmitting the access probe dueto a relatively large variation in signal RTT due to a large coveragearea on the surface of the earth, and the current position of thecommunications device not being known. However, the satellite's cellcoverage area may include, overlap with and/or be near a number ofsmaller cells, the smaller cells corresponding to terrestrial basestations. By approximating the communication device's current locationdetermined from terrestrial base station signals, the communicationsdevice may reduce the timing uncertainty as to when to transmit theaccess probe, thus increasing the likelihood that the access probe withbe accepted by the satellite base station, and reducing the time andnumber of different timing offset access probes that need to be sent tothe satellite BS. For example, the communications device may have storedinformation identifying the last terrestrial BS that the communicationsdevice used as an access point, the location of terrestrial BS beingknown and stored in the communications device, and informationcorrelating the terrestrial BS cells to the satellite position and/orsatellite cell location may also be stored and used. In someembodiments, the communications device may triangulate its positionbased on beacon signals received from a plurality of terrestrial basestations. In some embodiments, it may be possible to reduce the level oftiming uncertainty, by using location information derived fromterrestrial base stations, such that a first access probe signal to asatellite base station should be expected to hit an access slot of thesatellite base station.

Operation proceeds from either step 1708 to step 1710. In step 1710, thecommunications device codes information in a first access probe signalthat identifies a first superslot index. Then, in step 1712, thecommunications device transmits the first access probe signal thatidentifies a first superslot index, where the first access probe signalis transmitted at a first timing offset relative to the start of thefirst superslot index. Operation proceeds from step 1712 to step 1714,where the communications device monitors to determine if a response tothe first access probe signal was received from the base station. Then,in step 1716, operation proceeds to step 1718 if a response was notreceived or operation proceeds to step 1720 if a response was received.

If a response was received, then in step 1720, the communications deviceperforms a transmission timing adjustment as a function of informationincluded in the response.

However, if a response was not received, then in step 1718, thecommunications device codes information in a second access probe signalthat identifies a second superslot index and in step 1722 thecommunications device transmits the second access probe signal thatidentifies a second superslot index at a second timing offset relativeto the start of a second superslot having said second superslot index,the second timing offset being different than the first timing offset.Operation proceeds from step 1722 via connecting node A 1724 to step1726.

In step 1726, the communications device monitors to determine if aresponse to the second access probe signal was received from the basestation. Then, in step 1728, operation proceeds to step 1732 if aresponse was not received or operation proceeds to step 1730 if aresponse was received.

If a response was received, then in step 1732, the communications deviceperforms a transmission timing adjustment as a function of informationincluded in the response.

However, if a response was not received, then in step 1730, thecommunications device codes information in a third access probe signalthat identifies a third superslot index and in step 1734 thecommunications device transmits the third access probe signal at a thirdtiming offset relative to the start of a third superslot having saidthird superslot index, wherein the third timing offset is different fromthe first and second timing offsets.

Operation proceeds from step 1734 to step 1736. In step 1736, thecommunications device monitors to determine if a response to the thirdaccess probe signal was received from the base station. Then, in step1740, operation proceeds to step 1742 if a response was not received oroperation proceeds to step 1740 if a response was received.

If a response was received, then in step 1742, the communications deviceperforms a transmission timing adjustment as a function of informationincluded in the response. If a response was not received in step 1740,the communications device continues with the process of access signalgeneration/transmission/response determination/further action inaccordance with the embodiment. For example, in some embodiments, thecommunications device may communicate access probes with differenttiming offsets for each of successive access probes, until a probe isresponded to or until a fixed number of access probes have been sent.For example, the total number of access probes may be at least enough tocover the expected timing ambiguity.

In some embodiments, the first and second access probes are transmittedin different beacons slots and the second superslot index is the same ordifferent from the first superslot index. In some embodiments, the firstand second access probes are transmitted in different beacons slots andthe second superslot index is different from the first superslot index.

In some embodiments, the first and second access probes are transmittedin the same beaconslot, and the second superslot is different than thefirst superslot. In some such embodiments, the response includesinformation identifying the one of the probe signals being responded to.

In some embodiments, where a sequence including at least three accessprobes are transmitted, the second timing offset is different from thefirst timing offset by an initial timing offset value plus a firstinteger multiple of a fixed step size offset, and the third timingoffset is different from the first timing offset by the initial timingoffset value plus a second integer multiple of the fixed step sizetiming offset, which is different from the first integer multiple of thefixed step size offset. In some embodiments, the first and secondinteger multiples of the fixed step size timing offset can be eitherpositive or negative numbers.

In some embodiments, the fixed step size is less than the duration of abase station access interval, the base station access interval being aperiod of time during which the the base station is responsive to accessprobe signals.

In various embodiments, the base station to which the communicationsdevice is sending access probes is a satellite base station, and theround trip time (RTT) between the satellite base station and thecommunications device for signals traveling at the speed of light isgreater than the duration of a superslot. In some such embodiments, theRTT is also greater than the duration of a beaconslot. In someembodiments the RTT is greater then 0.2 seconds.

FIG. 18 is a flowchart 1800 of an exemplary method of operating anexemplary communications device in accordance with the presentinvention. The exemplary method of flowchart 1800 is a method ofoperating a communications device for use in a communications systemwhere beacon time slots occur on a periodic basis, a beacon signal beingtransmitted by a base station during each beacon time slot according toa periodic downlink timing structure, said downlink timing structureincluding a plurality of superslots within each beaconslot, theindividual superslots within a beacon slot being suitable foridentification through the use of a superslot index, each superslotincluding a plurality of symbol transmission time periods.

Operation starts in step 1802, where the communications device ispowered on and initialized. Operation proceeds from step 1802 to step1804, where the communications device is operated to receive at leastone beacon signal, and then in step 1806, the communications deviceprocesses the received beacon signal to determine a downlink timingreference point, superslots occurring within a beaconslot having apredetermined relationship to the determined timing reference point.Operation proceeds from step 1806 to step 1808.

In step 1808, the communications device codes in at least one of firstand second access probes an access probe identifier. In step 1810, thecommunications device transmits a first access probe at a timecorresponding to a first timing offset relative to the start of asuperslot in a beaconslot. Then, in step 1812, the communications devicetransmits a second access probe at a time corresponding to a secondtiming offset relative to the start of a superslot, the second accessprobe being transmitted at a point in time, which is less than thelarger of a superslot duration and twice the time required for atransmitted signal to travel from the communications device to the basestation, from the point in time at which the first access probe wastransmitted. Operation proceeds from step 1812 to step 1814.

In step 1814, the communications device is operated to monitor todetermine whether a response was received from the base station, and instep 1816 operation proceeds based upon the determination. If a responsewas received from the base station, operation proceeds from step 1816 tostep 1818. In step 1818 the communications device performs atransmission timing adjustment as a function of information included inthe response. If a response was not received from the base station,operation proceeds from step 1816 via connecting node A 1820 to step1804, where the communications device can restart the process ofinitiating access signaling.

In some embodiments, the maximum timing ambiguity is less than theduration of a superslot and the time between the transmission of thefirst and second access probes is less than the duration of a superslot.In some embodiments, the first and second access probes are transmittedat intervals from one another less than or equal to an access intervalduring which the base station will respond to received access probes.

In various embodiments, wherein the received response includesinformation identifying the access probe to which the responsecorresponds, the step of performing a transmission timing adjustment asa function of information included the response includes determining anamount of timing adjustment to be performed from timing correctioninformation received from the base station and information about thetransmission time of identified probe relative to the determineddownlink timing reference point.

FIG. 19 is a flowchart 1900 of an exemplary method of operating anexemplary communications device in accordance with the presentinvention. The exemplary method of flowchart 1900 is a method ofoperating a communications device for use in a communications system,e.g., an OFDM system, where beacon time slots occur on a periodic basis,a beacon signal being transmitted by a base station, e.g., satellitebase station, during each beacon time slot according to a periodicdownlink timing structure, said downlink timing structure including aplurality of superslots within each beaconslot, the individualsuperslots within a beacon slot being suitable for identificationthrough the use of a superslot index, each superslot including aplurality of symbol transmission time periods.

Operation starts in step 1902, where the communications device ispowered on and initialized. Operation proceeds from step 1902 to step1904, where the communications device is operated to receive at leastone beacon signal from the base station, and then in step 1906, thecommunications device processes the received beacon signal to determinea downlink timing reference point, superslots occurring within abeaconslot having a predetermined relationship to the determined timingreference point. Operation proceeds from step 1906 to step 1908.

In step 1908, the communications device is operated to transmit anaccess probe signal to a base station. Then, in step 1910, thecommunications device receives a response to the access probe signalfrom the base station, the response including information indicating atleast one of i) an mount of indicated main superslot timing offsetcorrection, the amount of main superslot correction being an integermultiple of a superslot time period; and ii) a superslot identifierindicating the position of a superslot within a beaconslot during whichthe base station received the access probe signal to which the receivedresponse corresponds. Operation proceeds from step 1910 to step 1912,where the communications device performs a timing adjustment as afunction of the information received in the received response. Step 1912includes sub-step 1914. In sub-step 1914, the communications devicedetermines a timing adjustment amount from information received from thebase station and information indicating the time the access probe signalwas transmitted.

In some embodiments, the received response from the base stationincludes a superslot identifier indicating the position of the superslotwithin a beacon slot during which the base station received the accessprobe signal and performing a transmission timing adjustment as afunction of information included in the response includes determining amain superslot timing offset from the superslot identifier included inthe received response and information indicating the superslot position,relative to the downlink timing reference point, within a beaconslot atwhich the access probe was transmitted, the main superslot timing offsetbeing an integer multiple of a duration of a superslot. In some suchembodiments, the received response further includes sub-superslot timingcorrection information including a sub-superslot time offset andperforming a transmission timing adjustment includes adjusting thetransmission timing by an amount corresponding to the sum of thedetermined main superslot timing offset and the sub-superslot timeoffset.

In various embodiments, the received response from the base stationincludes sub-superslot timing correction information indicating a mainsuperslot timing offset which is an integer multiple of a duration of asuperslot and a sub-superslot time offset which is a time offset that isless than the duration of a superslot. In some such embodiments, thestep of performing a transmission timing adjustment includes adjustingthe transmission timing by an amount corresponding to the sum of themain superslot timing offset and the sub-superslot time offset. In somesuch embodiments, the main superslot timing offset and sub-superslottime offset are communicated as part of a single coded value. In otherembodiments, the main superslot timing offset and the sub-superslot timeoffset are communicated as two separately coded values.

FIG. 20 is a flowchart 2000 of an exemplary method of operating awireless communications terminal in a system where base stations have adownlink timing structure that includes a plurality of superslots whichrecur in a periodic manner, each superslot including a plurality of OFDMsymbol transmission time periods. Operation starts in step 2002, wherethe wireless terminal is powered on and initialized. Operation proceedsfrom start step 2002 to step 2004, where the wireless terminal isoperated to determine if a base station to which the wireless terminalis seeking to send uplink signals is a satellite base station or aterrestrial base station. Based on the determination of step 2004,operation proceeds from step 2006 to either step 2008, in the case of asatellite BS or step 2010 in the case where the base station is aterrestrial base station.

In step 2008, the wireless terminal is operated to perform a firstuplink timing synchronization process, the first timing uplinksynchronization process supporting the communication of an uplink timingcorrection signal to the communications terminal. Step 2008 includessub-step 2012, 2014 and 2016. In sub-step 2012, the wireless terminal isoperated to transmit an access probe signal to the satellite basestation 2012. In step 2014, the wireless terminal is operated to receivea response to the access probe signal from the base station, theresponse including at least one of: i) an amount of an indicated mainsuperslot timing offset correction, the amount of the main superslottiming offset correction being an integer multiple of a superslot timeperiod; and ii) a superslot identifier indicating the position of asuperslot within a beaconslot during which the base station received theaccess probe signal to which the received response corresponds. Then instep 2016, the wireless terminal performs a transmission timingadjustment as a function of the information included in the receivedresponse.

In step 2010, the wireless terminal performs a second uplink timingsynchronization process, said second uplink timing synchronizationprocess being different from said first timing synchronization process.Step 2010 includes sub-step 2018, 2020 and 2022. In sub-step 2018, thewireless terminal transmits an access probe signal to the terrestrialbase station. In step 2018, the wireless terminal receives a response tothe access probe signal from the terrestrial base station, the responseincluding information indicating a timing correction which is less thanthe duration of a superslot. In some embodiments, the timing correctionis less than the duration of an access interval. In some embodiments,the timing correction is less than the duration of half an accessinterval. Then, in step 2022, the wireless terminal performs atransmission timing adjustment as a function of the information includedin the response received from the terrestrial base station, the timingadjustment involving changing the transmitter timing by an amount lessthan the duration of a superslot.

FIG. 21 is a drawing of an exemplary wireless terminal 2100, e.g.,mobile node, implemented in accordance with the present invention.Exemplary WT 2100 may be used in various embodiments of wirelesscommunications systems of the present invention. Exemplary WT 2100includes a receiver 2102, a transmitter 2104, a processor 2106, and amemory 2108 coupled together via a bus 2110 over which the variouselements may interchange data and information. The memory 2108 includesroutines 2120 and data/information 2122. The processor 2106, e.g., aCPU, executes the routines and uses the data information 2122 in memory2108 to control the operation of the WT 2100 and implement methods ofthe present invention.

Receiver 2102, e.g., an OFDM receiver, is coupled to a receive antenna2112 via which WT 2100 can receive downlink signals from a base stationincluding beacon signals and response signals including timingadjustment information. Transmitter 2104, e.g., an OFDM transmitter, iscoupled to a transmit antenna 2116 via which the WT 2100 can transmituplink signals to a base station including access probe signals. Timingof access probe signals including offsets from superslots, whichsuperslot and which beaconslot in which to transmit a given access probesignal is controllable in transmitter 2104. Receiver 2102 includes adecoder module 2114 used for decoding downlink signals, whiletransmitter 2104 includes an encoder module 2118 for encoding uplinksignals.

Routines 2120 includes a communications routine 2124 for implementingcommunications protocols used by the WT 2100 and WT control routines2125 for controlling operations of WT 2100. WT control routines 2125include a received signal processing module 2126, a coding module 2128,a transmitter control module 2130, a monitoring module 2132, a timingcorrection module 2134, a decoder module 2136, and a location basedtiming adjustment module 2138. Received signal processing module 2126processes signals including beacon signals and determines a downlinktiming reference point from at least one beacon signal. Coding module2128 operating, either alone or in conjunction with encoder 2118, insome embodiments, codes information in an access probe signal thatidentifies superslot index corresponding to the access probe signal. Insome embodiments, a WT identifier and/or a unique access probeidentifier is encoded and included in an access probe signal.Transmitter control module 2130 operates to control operations oftransmitter 2104 include controlling coded access probe signals to betransmitted with timing offsets, e.g., different timing offsets fordifferent access probes. In some embodiments, transmitter control module2130 controls the transmission of successive access probes to be greaterthan the twice the signaling time from the WT to the base station plus asignal processing time, e.g., allowing for the WT 2100 to see whether anaccess probe has been responded to before issuing another access probe.Monitoring module 2132 is used to determine if a response to an accessprobe signal is received from the base station. Timing correction module2134 is responsive to the monitoring module 2132 and performs atransmission timing adjustment as a function of information included ina received access probe response. Decoder module 2136 operating eitheralone or in conjunction with decoder 2114, decodes information in aresponse identifying the one of the access probe signals. Location basedtiming adjustment module 2138 determines a time at which to transmit afirst access probe as a function of location information determined froma signal received from a terrestrial base station. Location based timingadjustment module 2138 may be used to reduce the timing ambiguityassociated with a satellite base station due to a large coverage area,thus reducing the number of access probe needed and/or the average timeof the access process with the satellite base station.

Data/information 2122 includes timing/frequency structure information2140, user/device/session/resource information 2142, a plurality ofaccess probe signal information sets (1.sup.st access probe signal info2144, . . . , Nth access probe signal info 2146), received beacon signalinfo 2148, timing reference point information 2150, initial timingoffset information 2152, step size information 2154, received responsesignal information 2156, timing adjustment information 2158, andterrestrial BS/satellite BS location information 2160. Timing/frequencystructure information 2140 includes downlink and uplink timing andfrequency structure information, periodicity information, indexinginformation, OFDM symbol transmission time interval information,information regarding grouping of OFDM symbol transmission timeintervals such as slots, superslots, beaconslots, etc., base stationidentification information, beacon signal information, repetitiveinterval information, access interval information, uplink carrierfrequencies, downlink carrier frequencies, uplink tone blockinformation, downlink tone block information, uplink and downlink tonehopping information, base station identification information, etc.Timing/frequency structure information 2140 includes informationcorresponding to a plurality of base stations that may be in thewireless communications system. User/device/session/resource information2142 includes information corresponding to users of WT 2100, andinformation corresponding to peers in a communications session with WT2100, including, e.g., identifiers, addresses, routing information, airlink resources allocated, e.g., downlink traffic channel segments,uplink traffic channel segments for a multi-tone mode with terrestrialbase stations, a single dedicated logical tone for uplink signaling witha satellite BS, a base station assigned WT user identifier, etc.1.sup.st access probe information 2144 includes timing offsetinformation, e.g., relative to the start of a superslot, correspondingto the access probe, information identifying a superslot index, codedinformation, information identifying a beaconslot, etc. Nth access probeinformation 2146 includes timing offset information, e.g., relative tothe start of a superslot, corresponding to the access probe, informationidentifying a superslot index, coded information, informationidentifying a beaconslot, etc. Different sets of access probeinformation (2144, 2146) may include different information, eitherpartially or completely, e.g., different timing offsets, differentsuperslot index values or different timing offsets, the same superslotindex value. Access probe signal information (2144, 2146) may alsoinclude user identification information, e.g. a WT user identifierand/or a unique access probe signal identifier, and tone informationassociated with the access probe signal. Received beacon signalinformation 2148 includes information from a received beacon signal,e.g., information associating the beacon with a particular base station,carrier frequency, and/or sector, beacon signal strength information,information allowing the WT to establish a timing reference point, etc.Timing reference point information 2150 includes information, e.g.,determined using downlink beacon signaling, which establishes areference point, e.g., beaconslot start upon which superslot indexing isbased. Access probe signaling transmission timing can be referenced withrespect to the established timing reference point information 2150.Initial timing offset information 2152 includes information identifyingan initial timing offset value used in the calculation of timing offset,e.g., with respect to superslot start, for access probes. Step sizeinformation 2154 includes information identifying a fixed step sizetiming offset, which is added in integer multiples to the initial timingoffset, to determine the offset from the start of a superslot for aparticular access probe, e.g., with different access probes usingdifferent integer multiples of the step size timing offset. The fixedstep size is in some embodiments less than the duration of a basestation access interval, the base station access interval being a periodof time during which the base station is responsive to access probesignals. Received response signal information 2156 includes informationreceived in response to the access probe signaling including timingcorrection information. The timing correction information may be coded.In some embodiments, the response signal information 2156 also includesinformation identifying which one of the access probe signal is beingresponded to, e.g., via a WT identifier and/or a unique access probesignal identifier. Timing adjustment information 2158 includes timingcorrection information extracted from the received response signal andinformation indicating changes to the transmission timing as a result ofapplying the correction information. Terrestrial base station/satellitebase station location information 2160 includes information indicatingthe location of terrestrial base stations and the location of satellitebase stations in the system. Information 2160 may also includeinformation correlating the cell coverage areas or satellite basestations with terrestrial base stations.

FIG. 23 is a drawing of an exemplary wireless terminal 2300, e.g.,mobile node, implemented in accordance with the present invention.Exemplary WT 2100 may be used in various embodiments of wirelesscommunications systems of the present invention. Exemplary WT 2300includes a receiver 2302, a transmitter 2304, a processor 2306, and amemory 2308 coupled together via a bus 2310 over which the variouselements may interchange data and information. The memory 2308 includesroutines 2320 and data/information 2322. The processor 2306, e.g., aCPU, executes the routines and uses the data information 2322 in memory2308 to control the operation of the WT 2300 and implement methods ofthe present invention.

Receiver 2302, e.g., an OFDM receiver, is coupled to a receive antenna2312 via which WT 2300 can receive downlink signals from a base stationincluding beacon signals and response signals including timingadjustment information. Transmitter 2304, e.g., an OFDM transmitter, iscoupled to a transmit antenna 2316 via which the WT 2300 can transmituplink signals to a base station including access probe signals. Timingof access probe signals including offsets from superslots, whichsuperslot and which beaconslot in which to transmit a given access probesignal is controllable in transmitter 2304. Receiver 2302 includes adecoder module 2314 used for decoding downlink signals, whiletransmitter 2304 includes an encoder module 2318 for encoding uplinksignals.

Routines 2320 includes a communications routine 2324 for implementingcommunications protocols used by the WT 2300 and WT control routines2325 for controlling operations of WT 2300. WT control routines 2325include a received signal processing module 2326, a coding module 2328,a transmitter control module 2330, a C monitoring module 2332, a timingadjustment module 2334, and a decoder module 2136. Received signalprocessing module 2326 processes signals including beacon signals anddetermines a downlink timing reference point from at least one beaconsignal. Coding module 2328 operating, either alone or in conjunctionwith encoder 2318, in some embodiments, codes information in an accessprobe signal that identifies a corresponding access probe signal, e.g.,within a sequence of access probe signals. A wireless terminalidentifier and/or a unique access probe signal identifier may also beencoded to allow distinction between the plurality of WTs in the systemwhich may transmit access probes. Transmitter control module 2330operates to control operations of transmitter 2304 include controllingcoded access probe signals to be transmitted with timing offsets, e.g.,different timing offsets for different access probes. In someembodiments, the time between successive access probes may be less thanthe larger of the duration of a superslot and twice the time requiredfor a signal to travel from the WT to the base station. For example,consider that a superslot includes one access interval; however thetiming ambiguity may be greater than the access interval but less thanthe superslot duration, and the WT may transmit a sequence of accessprobes, e.g., coded to identify the access probe, spaced apart by a timeinterval less than the access interval to cover the possible timingrange ambiguity within the superslot. Monitoring module 2332 is used todetermine if a response to an access probe signal is received from thebase station. Timing adjustment module 2334 is responsive to themonitoring module 2332 and performs a transmission timing adjustment asa function of information included in a received access probe response.Decoder module 2336 operating either alone or in conjunction withdecoder 2314, decodes information in a response identifying the one ofthe access probe signals.

Data/information 2322 includes timing/frequency structure information2340, user/device/session/resource information 2342, a plurality ofaccess probe signal information sets (1.sup.st access probe signal info2344, . . . , Nth access probe signal info 2346), received beacon signalinfo 2348, timing reference point information 2350, access probespacing/offset information 2352, received response signal information2356, and timing adjustment information 2358. Timing/frequency structureinformation 2340 includes downlink and uplink timing and frequencystructure information, periodicity information, indexing information,OFDM symbol transmission time interval information, informationregarding grouping of OFDM symbol transmission time intervals such asslots, superslots, beaconslots, etc., base station identificationinformation, beacon signal information, repetitive interval information,access interval information uplink carrier frequencies, downlink carrierfrequencies, uplink block information, downlink tone block information,uplink and downlink tone hopping information, base stationidentification information, etc. Timing/frequency structure information2340 includes information corresponding to a plurality of base stationsthat may be in the wireless communications system.User/device/session/resource information 2342 includes informationcorresponding to users of WT 2300, and information corresponding topeers in a communications session with WT 2300, including, e.g.,identifiers, addresses, routing information, air link resourcesallocated, e.g., downlink traffic channel segments, uplink trafficchannel segments for a multi-tone mode with terrestrial base stations, asingle dedicated logical tone for uplink signaling with a satellite BS,a base station assigned WT user identifier, etc. 1.sup.st access probeinformation 2344 includes timing offset information, e.g., relative tothe start of a superslot, corresponding to the access probe, informationidentifying a superslot index, coded information, informationidentifying a beaconslot, etc. Nth access probe information 2346includes timing offset information, e.g., relative to the start of asuperslot, corresponding to the access probe, information identifying asuperslot index, coded information, information identifying abeaconslot, etc. Different sets of access probe information (2344, 2346)may include different information, either partially or completely, e.g.,different timing offsets but the same superslot. Access probe signalinformation (2344, 2346) may also include user identificationinformation, e.g. a WT identifier and/or a unique access probe signalidentifier, and tone information associated with the access probesignal. Received beacon signal information 2348 includes informationfrom a received beacon signal, e.g., information associating the beaconwith a particular base station, carrier frequency, and/or sector, beaconsignal strength information, information allowing the WT to establish atiming reference point, etc. Timing reference point information 2350includes information, e.g., determined using downlink beacon signaling,which establishes a reference point, e.g., beaconslot start upon whichsuperslot indexing is based. Access probe signaling transmission timingcan be referenced with respect to the established timing reference pointinformation 2350. Access probe spacing/offset information 2352 includestiming information relating to access probes in a sequence of accessprobes, e.g., a delta time interval between successive access probes.For example, in a case where each an access interval duration is lessthan a superslot, but the timing ambiguity is greater than an accessinterval duration, a number of successive access probes may be spaced bya delta time interval less than or equal to the access intervalduration, and the number being such to cover the timing ambiguity range.Received response signal information 2356 includes information receivedin response to the access probe signaling including timing correctioninformation. The timing correction information may be coded. In someembodiments, the response signal information 2356 also includesinformation identifying which one of the access probe signals in thesequence of successive access probes is being responded to. Timingadjustment information 2358 includes timing correction informationextracted from the received response signal and information indicatingchanges to the transmission timing as a result of applying thecorrection information. Received response signal information 2356 mayalso include a WT identifier and/or a unique access probe signalidentifier.

FIG. 24 is a drawing of an exemplary wireless terminal 2400, e.g.,mobile node, implemented in accordance with the present invention.Exemplary WT 2400 may be used in various embodiments of wirelesscommunications systems of the present invention. Exemplary WT 2400includes a receiver 2402, a transmitter 2404, a processor 2406, and amemory 2408 coupled together via a bus 2410 over which the variouselements may interchange data and information. The memory 2408 includesroutines 2420 and data/information 2422. The processor 2406, e.g., aCPU, executes the routines and uses the data information 2422 in memory2408 to control the operation of the WT 2400 and implement methods ofthe present invention.

Receiver 2402, e.g., an OFDM receiver, is coupled to a receive antenna2412 via which WT 2400 can receive downlink signals from a base stationincluding beacon signals and response signals including timingadjustment information. Transmitter 2404, e.g., an OFDM transmitter, iscoupled to a transmit antenna 2416 via which the WT 2400 can transmituplink signals to a base station including access probe signals. Timingof access probe signals including offsets from superslots, whichsuperslot and which beaconslot in which to transmit a given access probesignal is controllable in transmitter 2404. Receiver 2402 includes adecoder module 2414 used for decoding downlink signals, whiletransmitter 2404 includes an encoder module 2418 for encoding uplinksignals.

Routines 2420 includes a communications routine 2424 for implementingcommunications protocols used by the WT 2400 and WT control routines2425 for controlling operations of WT 2400. WT control routines 2425include a received signal processing module 2426, a coding module 2428,a transmitter control module 2430, a monitoring module 2432, atransmission timing adjustment module 2434, and a receiver control anddecoding module 2436. Received signal processing module 2426 processessignals including beacon signals and determines a downlink timingreference point from at least one beacon signal. Coding module 2128operating, either alone or in conjunction with encoder 2118, codesinformation in uplink signals, e.g., encoding a WT identifier and/or aunique access probe identifier in an access probe signal to betransmitted by WT 2400, allowing the access probe to be distinguished bythe BS from other access probes which may have been transmitted by otherWTs. Transmitter control module 2430 operates to control operations oftransmitter 2404 include controlling access probe signals to betransmitted with timing offsets, e.g., different timing offsets from thestart of a superslot for different access probes. In some embodiments,transmitter control module 2430 controls the transmission of successiveaccess probes to be greater than the twice the signaling time from theWT to the base station plus a signal processing time, e.g., allowing forthe WT 2400 to see whether an access probe has been responded to beforeissuing another access probe. Monitoring module 2432 is used todetermine if a response to an access probe signal is received from thebase station. Transmission timing adjustment module 2434 is responsiveto the monitoring module 2432 and performs a transmission timingadjustment as a function of information included in a received accessprobe response signal. For example, the transmission timing adjustmentmodule 2434 may use the information in the received response signal,e.g., sub-superslot timing offset correction information 2464, and oneof a main superslot timing offset correction value or a superslotposition indicator indicative of reception in the base station, inconjunction with information known to the WT 2400 as to when the accessprobe was transmitted, to calculate a timing adjustment. In someembodiments, the received response signal conveys sub-superslot timingoffset information, e.g., via coded bits in the response signal, andmain superslot timing offset information is conveyed via the time oftransmission of the response signal. In some embodiments, receivercontrol and decoder module 2436 operating either alone or in conjunctionwith decoder 2414, receives an access probe response signal from thebase station and decodes information in a response extracting at leastone of i) an amount of an indicated main superslot timing offsetcorrection, the amount of the main superslot timing offset correctionbeing an integer multiple of a superslot time period; and ii) asuperslot identifier indicating the position of a superslot within abeacon slot during which the base station received the access probesignal to which the received response corresponds. In some embodiments,a main superslot timing offset has been coded with a sub-superslot timeoffset as a single coded value and module 2436 performs the decodingoperation. In some embodiments, a main superslot timing offset has beencoded separately from a sub-superslot time offset as a two separatelycoded values and module 2436 performs the decoding operation. In someembodiments, sub-slot timing offset is conveyed via coded bits of theresponse signal and main superslot offset is conveyed via controllingthe time of transmission of the response signal, e.g., within theresponse signal being offset by different amounts. In some embodimentsthe response signal also includes a WT identifier and/or a unique accessprobe signal identifier 2465 such that the WT 2400 can recognize thatthe response signal is directed to the WT 2400 and not to another WT inthe system.

Data/information 2422 includes timing/frequency structure information2440, user/device/session/resource information 2442, a plurality ofaccess probe signal information sets (1.sup.st access probe signal info2444, . . . , Nth access probe signal info 2446), received beacon signalinfo 2448, timing reference point information 2450, initial timingoffset information 2452, step size information 2454, received responsesignal information 2456, and timing adjustment information 2458.Timing/frequency structure information 2440 includes downlink and uplinktiming and frequency structure information, periodicity information,indexing information, OFDM symbol transmission time intervalinformation, information regarding grouping of OFDM symbol transmissiontime intervals such as slots, superslots, beaconslots, etc., basestation identification information, beacon signal information,repetitive interval information, access interval information, uplinkcarrier frequencies, downlink carrier frequencies, uplink tone blockinformation, downlink tone block information, uplink and downlink tonehopping information, base station identification information, etc.Timing/frequency structure information 2440 includes informationcorresponding to a plurality of base stations that may be in thewireless communications system. User/device/session/resource information2442 includes information corresponding to users of WT 2400, andinformation corresponding to peers in a communications session with WT2400, including, e.g., identifiers, addresses, routing information, airlink resources allocated, e.g., downlink traffic channel segments,uplink traffic channel segments for a multi-tone mode with terrestrialbase stations, a single dedicated logical tone for uplink signaling witha satellite BS, a base station assigned WT user identifier, etc.1.sup.st access probe information 2444 includes timing offsetinformation, e.g., relative to the start of a superslot, correspondingto the access probe, information identifying a superslot index,information identifying a beaconslot, etc. Nth access probe information2446 includes timing offset information, e.g., relative to the start ofa superslot, corresponding to the access probe, information identifyinga superslot index, information identifying a beaconslot, etc. Differentsets of access probe information (2444, 2446) may include differentinformation, either partially or completely, e.g., different timingoffsets, different superslot index values or different timing offsets,the same superslot index value. Access probe signal information (2444,2446) may also include user identification information, e.g., a WTidentifier and/or a unique access probe signal identifier, and toneinformation associated with the access probe signal. The WT identifierand/or unique access probe signal identifier may be encoded into theaccess probe signal such that the BS can distinguish among a pluralityof access probes, e.g., from different WTs in the system, and the BS mayinclude identification in response signals allowing WT 2400 to know thata response signal is directed to WT 2400. Received beacon signalinformation 2448 includes information from a received beacon signal,e.g., information associating the beacon with a particular base station,carrier frequency, and/or sector, beacon signal strength information,information allowing the WT to establish a timing reference point, etc.Timing reference point information 2450 includes information, e.g.,determined using downlink beacon signaling, which establishes areference point, e.g., beaconslot start upon which superslot indexing isbased. Access probe signaling transmission timing can be referenced withrespect to the established timing reference point information 2450.Initial timing offset information 2452 includes information identifyingan initial timing offset value used in the calculation of timing offset,e.g., with respect to superslot start, for access probes. Step sizeinformation 2454 includes information identifying a fixed step sizetiming offset, which is added in integer multiples to the initial timingoffset, to determine the offset from the start of a superslot for aparticular access probe, e.g., with different access probes usingdifferent integer multiples of the step size timing offset. The fixedstep size is in some embodiments less than the duration of a basestation access interval, the base station access interval being a periodof time during which the base station is responsive to access probesignals. Received response signal information 2456 includes informationreceived in response to the access probe signaling including timingcorrection information. Received response signal information 2456 mayinclude a WT identifier and/or a unique access probe signal identifier2465, allowing the WT 2400 to recognize that the response signal itdirected to itself and not to another WT in the system. In someembodiments, the response signal information 2156 also includesinformation identifying which one of the access probe signalstransmitted by WT 2400 is being responded to, e.g., if the WT 2400transmits a plurality of access probes in a time interval less thantwice the signal transmit time from the WT to BS. The timing correctioninformation may be coded. In some embodiments, the response signalinformation 2156 also includes information identifying which one of theaccess probe signal is being responded to. Received response signalinformation 2456 includes a sub-superslot timing offset correctioninformation 2464, and, in some embodiments, at least one of a mainsuperslot timing offset correction information 2460, e.g., an integermultiple of a superslot time period, and a superslot position identifier2462, e.g., identifying the position of a superslot within a beaconslotduring which the base station received the access probe signal to whichthe received response corresponds. Timing adjustment information 2458includes timing correction information extracted from the receivedresponse signal and information indicating changes to the transmissiontiming as a result of applying the correction information, e.g., incombination with know timing information corresponding to the accessprobe.

FIG. 22 illustrates a method of operating a base station, e.g, asatellite base station, in accordance with one exemplary embodiment ofthe invention. All or portions of the method may be used depending onthe particular embodiment and type of wireless terminal signaling sentto the base station, e.g., the type of information encoded ontransmitted access probes.

The method starts in step 2202, e.g., with the base station beinginitialized and placed into operation. Operation proceeds along parallelpaths to steps 2203 and 2204 which may be performed in parallel. In step2203 the base station transmits beacon signals on a periodic basisaccording to a predetermined downlink timing structure, at least onebeacon signal being transmitted during each beacon slot. The beaconsignal, in various embodiments, is a signal transmitted at a higherpower level than is normally used to transmit user data, e.g., text,video or application data. The beacon signal, in some embodiments is anarrowband signal. In some embodiments a beacon signal is implemented asa single tone signal which is transmitted for one a few consecutivesymbol transmission time periods, e.g., less than 3 or 4 consecutiveOFDM symbol time periods. The beacon signals are transmitted on aperiodic basis as determined by the downlink timing structure.

In step 2204, which can occur in parallel with the beacon transmissionstep 2203, the base station monitors, e.g., during access intervalswhich occur on a periodic basis, to detect access probe signals. In someembodiments, the periodic access intervals have a duration shorter thanthe period of a downlink superslot. The access probe signals may bereceived from one or more communications devices which have not yetfully achieved uplink timing synchronization with the base station.Superslot and/or sub-superslot uplink timing corrections may be requiredbefore the wireless terminals sending the access probes will achievesymbol level uplink timing synchronization with the base station. Foreach access probe signal detected in step 2204, operation proceeds tostep 2206, In step 2206, the base station determines the index of adownlink superslot time period at said base station during which theaccess probe signal was received. This may be different from thesuperslot in which the transmitting communications device believed itwas transmitting the access probe in. The determination of whichdownlink superslot an access probe signal was received in can be doneusing internal base station timing information and knowledge of when theaccess probe was received.

Operation proceeds from step 2206 to step 2208. In step 2208, the basestation performs a decoding operation on the access probe signal todetect information that may have been encoded on the signal, e.g., anaccess probe identifier, communications device identifier whichidentifies the transmitting communication device, and/or a downlinksuperslot identifier indicating for example, an index of a superslotwithin a beacon slot in which the transmitting device sent the accessprobe.

With the access probe information having been decoded, operation proceesto steps 2210 and 2212. In step 2210 the base station determines asub-superslot uplink transmission timing correction offset to be made bythe communications device which transmitted the received probe toachieve proper symbol level timing within a superslot for signals, e.g.,OFDM symbols, transmitted to the base station. This timing correctionvalue is a value which indicates a correction which is less than theduration of a superslot. Operation proceeds form step 2210 to step 2214.

Step 2212 is an optional step performed in some embodiments where asuperslot index is encoded on the received access probe. In step 2212which is performed in some but not necessarily all embodiments, a mainsuperslot timing offset correction is determined from the differencebetween the determined index of the downlink superslot in which theaccess probe was received and the index of the superslot in which theaccess probe was transmitted as indicated by the decoded superslotidentifier. Operation proceeds from step 2212 to step 2214.

Step 2214 is a step in which a response to the received access probe isgenerated and transmitted. In some embodiments, the response istransmitted in a downlink superslot having a predetermined downlinksuperslot offset from the downlink superslot time period in which theaccess probe to which the response corresponds was received by the basestation. The superslot offset is sufficient for the base station toprocess and generate the necessary response, e.g., one or two superslotsfrom the superslot in which the access probe was received. Such anembodiment, which transmits responses in a downlink superslot having apredetermined known superslot offset from the superslot in which aresponse was received allows a wireless terminal to estimate thesuperslot timing offset error from the response timing.

In some embodiments where access probe response signals transmit theresponse at a predetermined superslot offset to the point in time inwhich the access probe is received, the wireless terminal receiving theaccess probe response calculates a main timing adjustment to beimplemented according to the following equation:

main timing adjustment=2.times.(index of superslot in which the responseto the access probe was received−index of superslot determined by thecommunications device in which the access probe was transmitted)−a fixedsuperslot delay) times the period of a superslot. The fixed superslotdelay is a function of the predetermined offset. The 2 multiplier takesinto consideration that the delay involved is a round trip delay whilethe multiplication times the period of a downlink superslot takes intoconsideration the duration of superslots.

In step 2214, the sub-superslot uplink timing offset correctiondetermined in step 2210 is encoded into the response. In addition, otherinformation may also be encoded into the access probe response signalwhich is generated. Each of the elements may be coded separately, e.g.,as separate error values or may be combined, e.g., with main andsub-superslot error information being coded as a single value. Insub-step 2224, the main superslot uplink timing offset correction, e.g.,the correction value generated in step 2212, is coded into the responsesignal. In sub-step 226, the superslot identifier indicating the indexof the downlink superslot in which the access probe signal was receivedis encoded into the response signal. In sub-step 2228. thecommunications device identifier and/or access probe identifiercorresponding to the received access probe which is being responded tois encoded into the response signal. Identification of thecommunications device to which the response is directed can be useful ina multi-user system particularly where multiple devices may makerequests, e.g., as part of a contention based access process. Operationproceeds from step 2214 to step 2230 where the generated probe istransmitted as an access probe response signal. Processing correspondingto the received detected access probe stops in step 2232 however, thereceipt and processing of other access probes may continue.

FIG. 25 is a drawing of an exemplary base station 2500, e.g., asatellite based base station, implemented in accordance with the presentinvention and using methods of the present invention. Exemplary basestation 2500 may be the BS of an exemplary wireless communicationssystem, implemented in accordance with the present invention. The basestation 2500 is sometimes referred to an access node, as the basestation provides network access to WTs. The base station 2500 includes areceiver 2502, a transmitter 2504, a processor 2506, and a memory 2508coupled together via a bus 2510 over which the various elements mayinterchange data and information. The receiver 2502 includes a decoder2512 for decoding received uplink signals from WTs, e.g., includingaccess probe signals. The transmitter 2504 includes an encoder 2514 forencoding downlink signals to be transmitted to WTs, e.g., includingdownlink beacon signals and downlink response signals to access probes.The receiver 2502 and transmitter 2504 are each coupled to antennas(2516, 2518) over which uplink signals are received from WTs anddownlink signals are transmitted to WTs, respectively. In someembodiments, the same antenna is used for the receiver 2502 andtransmitter 2504. In addition to communicating with WTs, the basestation 2500 can communicate with other network nodes. In someembodiments where the BS 2500 is a satellite BS the BS communicates witha ground station with a directional antenna and high capacity link, theground station coupled to other network nodes, e.g., other basestations, routers, AAA servers, home agent nodes and the Internet. Insome such embodiments, the same receivers 2502, transmitters 2504,and/or antennas previously described with BS—WT communication links areused for BS—network node ground station links, while in otherembodiments separate elements are used for different functions. Inembodiments, where the BS 2500 is a terrestrial base station, BS 2500includes a network interface which couples the BS 2500 to other networknodes and/or the Internet. The memory 2508 includes routines 2520 anddata/information 2522. The processor 2506, e.g., a CPU, executes theroutines 2520 and uses the data/information 2522 in memory 2508 tocontrol the operation of the base station 2500 and implement the methodsof the present invention.

The memory 2508 includes a communications routine 2524 and base stationcontrol routine 2526. The communications routine 2524 implements thevarious communications protocols used by the base station 2500. The basestation control routine 2526 includes a scheduler module 2528, whichassigns segments, e.g., downlink traffic channel segments, to WTs, atransmitter control module 2530, a receiver control module 2536, anencoder module 2546, an access probe decoding and processing module2548, and a timing correction determination module 2550.

Transmitter control module controls operation of transmitter 2504. Thetransmitter control module 2530 includes a beacon module 2532 and anaccess probe response module 2534. Beacon module controls transmissionof beacon signals, e.g., the transmission of at least one beacon signalduring a beaconslot. In some embodiments, the beacon signal is a singletone signal. In some embodiments, the beacons signal has a duration ofless than three OFDM symbol transmission time periods. Access proberesponse module 2542 controls the generation and transmission ofresponse signals, which are responding to access probe signals.

The receiver control module 2536 includes an access probe reception anddetection module 2540. Receiver control module 2536 controls thereceiver 2502 operation. Access probe reception and detection module2540 is used in receiving and detecting access probe signals fromwireless terminals. The access probe detection module 2540 includes anaccess probe detection module 2542 and an access time intervaldetermination module 2544. Access time interval determination module2544 identifies the predetermined periodic time periods occurring duringa portion of each superslot during a beaconslot, said portion being lessthan one half of a superslot, the predetermined time periods sometimesreferred to as access intervals or slots being reserved for receivingaccess probes. Access probes arriving outside the access intervals aretreated by the base station as interference and not responded to. Insome embodiments, an access interval is less than 25% of a superslotinterval. For example, an access interval may be 8 or 9 OFDM symboltransmission time intervals corresponding to a superslot of 114 OFDMsymbol transmission time intervals. In some embodiments, an OFDM symboltransmission time interval is approximately 100 micro-sec. Access probedetection module 2542 detects and processes received access probes whicharrive during time intervals deemed acceptable by the access timeinterval determination module 2544.

Encoder module 2546, operating either alone or in conjunction withencoder 2514, in some embodiments, includes in the response signal asuperslot identifier indicating the position of the superslot within abeaconslot during which the base station received the access probesignal. In some embodiments, the encoder module, operating either aloneor in conjunction with encoder 2514, encodes sub-superslot timingcorrection information in the response signal, said super-slot timingcorrection information indicating a timing adjustment smaller than theduration of a superslot.

Access probe decoding and processing module 2548, operating either aloneor in conjunction with decoder 2512, decodes received access probesignals to recover encoded information, e.g., an encoded superslotidentifier, encoded information identifying a WT, encoded informationidentifying the access probe signal.

In some embodiments, timing correction determination module 2550determines a main superslot timing offset correction, e.g., an integermultiple of the duration of a superslot from the difference between thedecoded superslot identifier and the superslot index within a beaconslotof the superslot in which the access probe was received. In someembodiments, timing correction determination 2550 determines a mainsuperslot timing offset correction based on a beacon transmissionreference point, and a reference point of the received access probesignal. In some such embodiments, the access probe signal does conveyinformation identifying the index of the superslot during which the WTtransmitted the access probe signal. In some such embodiments, theresponse signal conveys timing adjustment information which is combinedby the WT with access signal offset information known to the WT, but notknown to the BS. In some such embodiments, a sub-superslot timingcorrection is conveyed in the response signal via coded bits while themain timing offset information is conveyed by the transmission time ofthe response signal.

Data/information 2522 includes user data/information 2552 which includesa plurality of sets of information (user 1/MN session A session Bdata/information 2554, user N/MN session X data/information 2556)corresponding to the wireless terminals using the base station 2500 astheir point of network attachment. Such WT information may include,e.g., WT identifiers, routing information, assigned uplink singlelogical tone, downlink segment assignment information, userdata/information, e.g., voice information, data packets of text, video,music, etc., coded blocks of information. Data/information 2522 alsoincludes system information 2574 including downlink/uplink timing andfrequency structure information 2576, beacon signal information 2558,received access probe signal information 2560 and response signalinformation 2562. The response signal information includes sub-superslottiming offset correction information 2572, and at least one of mainsuperslot timing offset correction information 2564, superslotidentifier information 2566, communications device identifierinformation 2568, and access probe identifier information 2570.

In some embodiments, the main superslot timing offset correction is aninteger multiple of a superslot time period. A superslot identifier canbe used to indicate the position of the superslot within a beaconslotduring which the base station received the access probe signal to whichthe received response corresponds. A communications device identifiercan be used to identify the communications device which transmitted theaccess probe signal to which the received response corresponds. Anaccess probe identifier can be used to identify the access probe towhich the response signal corresponds.

Downlink/uplink timing and frequency structure information 2576including OFDM symbol transmission timing information, informationcorresponding to grouping of OFDM symbols, e.g., slot, superslot,beaconslot, access interval, etc. information, beacon timing and toneinformation, indexing information, e.g., of superslots within abeaconslot, carrier frequencies used for uplink and downlink, toneblocks used for uplink and downlink, tone hopping information for uplinkand downlink, timing relationships and offsets between uplink anddownlink timing structure at the base station, periodic intervals withinthe timing structures, etc.

The techniques of the present invention may be implemented usingsoftware, hardware and/or a combination of software and hardware. Thepresent invention is directed to apparatus, e.g., mobile nodes such asmobile terminals, base stations, communications system which implementthe present invention. It is also directed to methods, e.g., method ofcontrolling and/or operating mobile nodes, base stations and/orcommunications systems, e.g., hosts, in accordance with the presentinvention. The present invention is also directed to machine readablemedium, e.g., ROM, RAM, CDs, hard discs, etc., which include machinereadable instructions for controlling a machine to implement one or moresteps in accordance with the present invention.

In various embodiments nodes described herein are implemented using oneor more modules to perform the steps corresponding to one or moremethods of the present invention, for example, signal processing,message generation and/or transmission steps. Thus, in some embodimentsvarious features of the present invention are implemented using modules.Such modules may be implemented using software, hardware or acombination of software and hardware. Many of the above describedmethods or method steps can be implemented using machine executableinstructions, such as software, included in a machine readable mediumsuch as a memory device, e.g., RAM, floppy disk, etc. to control amachine, e.g., general purpose computer with or without additionalhardware, to implement all or portions of the above described methods,e.g., in one or more nodes. Accordingly, among other things, the presentinvention is directed to a machine-readable medium including machineexecutable instructions for causing a machine, e.g., processor andassociated hardware, to perform one or more of the steps of theabove-described method(s).

The timing synchronization methods and apparatus of the presentinvention can be used with a wide variety of devices and systems. Themethods and apparatus of the present invention are well suited for use,and can be used in combination with the methods and apparatus describedin U.S. Utility patent application Ser. No. 11/184,051, titled“COMMUNICATIONS SYSTEM, METHODS AND APPARATUS” which is filed on thesame day as the present application and names the same inventors as thepresent application. This utility patent application is hereby expresslyincorporated by reference and is to be deemed as part of the disclosureof the present patent application.

While described in the context of an OFDM system, at least some of themethods and apparatus of the present invention, are applicable to a widerange of communications systems including many non-OFDM and/ornon-cellular systems.

Numerous additional variations on the methods and apparatus of thepresent invention described above will be apparent to those skilled inthe art in view of the above description of the invention. Suchvariations are to be considered within the scope of the invention. Insome embodiments the base stations server as access nodes whichestablish communications links with mobile nodes (WTs) using OFDMsignals. In various embodiments the WTs are implemented as cell phones,notebook computers, personal data assistants (PDAs), or other portabledevices including receiver/transmitter circuits and logic and/orroutines, for implementing the methods of the present invention.

What is claimed is:
 1. A method of operating a communications device foruse in a communications system Where beacon time slots occur on aperiodic basis, a beacon signal being transmitted by a satellite basestation during each beacon time slot according to a periodic downlinktiming structure, said downlink timing structure including a pluralityof superslots within each beacon slot, the individual superslots withina beacon slot being identifiable through the use of a superslot index,each superslot including a plurality of symbol transmission timeperiods, the method comprising: receiving at least one beacon signalfrom the satellite base station; processing the received beacon signalto determine a downlink timing reference point, superslots occurringwithin a beacon slot having a predetermined relationship to thedetermined downlink timing reference point; transmitting an access probesignal to said base station; receiving a response to the access probesignal from the base station, the response including informationindicating at least one of: i) an amount of an indicated main superslottiming offset correction, the amount of the main superslot timing offsetcorrection being an integer multiple of a superslot time period; ii) asuperslot identifier indicating the position of a superslot within abeacon slot during which the base station received the access probesignal to which the received response corresponds; iii) an identifierthat identifies the communications device which transmitted the accessprobe signal to which the received response corresponds or iv) anidentifier that identifies the access probe to which the response beingresponding to; and performing a transmission timing adjustment as afunction of the information included in the received response.
 2. Themethod of 1, wherein said identifier of the received access probeincludes information that identifies the downlink superslot in which thecommunication device determined the probe was transmitted.
 3. The methodof claim 1, wherein said received response is transmitted to thecommunications device at a fixed predetermined superslot time offsetfrom the time it was received by the base station.
 4. The method ofclaim 1, wherein said access probe signal is an OFDM signal.
 5. Themethod of claim 4, wherein said beacon signal is a single tone signal.6. The method of claim 1, wherein the received response from the basestation includes a superslot identifier indicating the position of thesuperslot within a beacon slot during which the base station receivedthe access probe signal, and wherein performing a transmission timingadjustment as a function of information included in said responseincludes: determining a main supers lot timing offset from the superslotidentifier included in the received response and information indicatingthe superslot position, relative to said downlink timing referencepoint, within a beacon slot at which said access probe was transmitted,the main superslot timing offset being an integer multiple of theduration of a superslot.
 7. The method of claim 6, wherein the receivedresponse further includes sub-superslot timing correction informationincluding a sub-superslot time offset; and wherein performing atransmission timing adjustment includes adjusting said transmissiontiming by an amount corresponding to the sum of the determined mainsuperslot timing offset and said sub-superslot time offset.
 8. Themethod of claim 1, wherein the received response from the base stationincludes information indicating a main supers lot timing offset which isan integer multiple of the duration of a superslot and a sub-superslottime offset which is a time offset that is less than the duration of asuperslot.
 9. The method of claim 8, wherein said main superslot timingoffset and said sub-superslot time offset are communicated as part of asingle coded value.
 10. The method of claim 8, wherein said mainsuperslot timing offset and said sub-superslot time offset arecommunicated as two separately coded values.
 11. The method of claim 8,wherein performing a transmission timing adjustment includes adjustingsaid transmission timing by an amount correspond to the sum of the mainsuperslot timing offset and said sub-superslot time offset.
 12. Themethod of claim 1, wherein performing a transmission timing adjustmentincludes determining a timing adjustment amount from the location in abeaconslot of the superslot which included the received access proberesponse and information about the time the access probe signal wastransmitted.
 13. The method of claim 12, wherein said informationindicating the time the access probe signal was transmitted indicatesthe index of a downlink superslot as determined from said determinedreference point in time in which said access probe, to which a responsewas received, was transmitted.
 14. The method of claim 1, whereinperforming a transmission timing adjustment includes determining a maintiming adjustment amount from information received from said basestation and information indicating a superslot index, determined by saidcommunications device to be the index within a beacon slot at which theprobe being responded to was transmitted.
 15. The method of claim 14,wherein said main timing adjustment is a timing adjustment correspondingto an integer multiple of a superslot time period, determining the maintiming adjustment further being performed as a function of a fixedsuperslot delay between the time the base station receives an accessrequest and transmits a response to the received access request.
 16. Themethod of claim 14, wherein the main timing adjustment is determinedaccording to the following equation:main timing adjustment =2×(index of superslot in which the response tothe access probe was received−index of superslot determined by thecommunications device in which the access probe was transmitted)−a fixedsuperslot delay) times the period of a superslot.
 17. The method ofclaim 16 wherein performing a transmission timing adjustment furtherincludes: adjusting said timing by a sub-superslot offset specified bysaid base station in said response to the transmitted access request.18. A communications device for use in a communications system wherebeacon time slots occur on a periodic basis, a beacon signal beingtransmitted by a satellite base station during each beacon time slotaccording to a periodic downlink timing structure, said downlink timingstructure including a plurality of superslots within each beacon slot,the individual superslots within a beacon slot being identifiablethrough the use of a superslot index, each superslot including aplurality of symbol transmission time periods, the method comprising: areceiver configured to receive at least one beacon signal from thesatellite base station; a processing module configured to process thereceived beacon signal to determine a downlink timing reference point,superslots occurring within a beacon slot having a predeterminedrelationship to the determined downlink timing reference point; atransmitter module for transmitting an access probe signal to said basestation; a receiver module for receiving a response to the access probesignal from the base station, the response including informationindicating at least one of: i) an amount of an indicated main superslottiming offset correction, the amount of the main superslot timing offsetcorrection being an integer multiple of a superslot time period; ii) asuperslot identifier indicating the position of a superslot within abeacon slot during which the base station received the access probesignal to which the received response corresponds, iii) an identifierthat identifies the communications device which transmitted the accessprobe signal to which the received response corresponds or iv) anidentifier that identifies the access probe to which the response beingresponding to; and a transmission timing adjustment module forperforming a transmission timing adjustment as a function of theinformation included in the received response.
 19. The communicationsdevice of aim 18, wherein said access probe signal is an OFDM signal.20. The communications ice of claim 19, wherein said beacon signal is asingle tone signal.
 21. A method of operating a communications terminalin an OFDM system including terrestrial and satellite base stationswhich use OFDM signals for both uplink and downlink signaling, themethod comprising: determining if a base station to which saidcommunications terminal is seeking to send OFDM uplink signals is asatellite base station or a terrestrial base station; performing a firstuplink timing synchronization process if it is determined that said basestation is a satellite base station; performing a second uplink timingsynchronization process if it is determined that said base station is aterrestrial base station, said second uplink timing synchronizationprocess being different from said first uplink timing synchronizationprocess; wherein both said terrestrial and satellite base stations havea downlink timing structure that includes a plurality of superslotswhich recur in a periodic manner, each superslot including a pluralityof OFDM symbol transmission time periods; and wherein said first uplinktiming synchronization process supports the communication of an uplinktiming correction signal to the communications terminal which indicatesan uplink transmission timing synchronization correction exceeding theduration of a downlink superslot.
 22. The method of claim 21, whereinperforming the first uplink timing synchronization process includes:transmitting an access probe signal to said satellite base station;receiving a response to the access probe signal from the base station,the response including information indicating at least one of i) anamount of an indicated main superslot timing offset correction, theamount of the main superslot timing offset correction being an integermultiple of a superslot time period; or ii) a superslot identifierindicating the position of a superslot within a beacon slot during whichthe base station received the access probe signal to which the receivedresponse corresponds; and performing an uplink transmission timingadjustor as a function of the information included in the receivedresponse.
 23. The method of claim 21, wherein performing the seconduplink timing synchronization process includes: transmitting an accessprobe signal to said terrestrial base station; receiving a response tothe access probe signal from the terrestrial base station, the responseincluding information indicating a timing correction which is less thanthe duration of a superslot; and performing a transmission timingadjustment as a function of the information included in the responsereceived from the terrestrial base station, the timing adjustmentinvolving changing transmitter timing by an amount less then theduration of a superslot.
 24. A method of operating a wireless terminalto communicate with base stations in a communications system including aplurality of base stations, the method comprising: storing base stationlocation information in memory; receiving a signal from a first basestation; determining a time at which to transmit an access signal to asecond base station as a function of stored location informationindicating the location of said first base station from which saidwireless terminal received said signal, said determining a time at whichto transmit an access signal to a second base station includingreceiving a signal from the second base station, determining from thereceived signal a timing reference point, and determining said time atwhich to transmit an access signal relative to the determined referencepoint using uplink timing correction information calculated from saidstored location information indicating the location of said first basestation and information indicating the location of said second basestation, said time at which to transmit an access signal beingdetermined so that the transmitted access signal will be received at thesecond base station during an access interval in which received accesssignals are processed by said second base station; and transmitting saidaccess signal at the determined time.
 25. The method of claim 24,wherein said first base station is a terrestrial base station and saidsecond base station is a satellite base station.
 26. The method of claim25, wherein said access signal is an OFDM signal and wherein said signalreceived from said second base station is a beacon signal.
 27. Themethod of claim 25, wherein said first and second base stations each usea periodic downlink timing structure, said downlink timing structureincluding beacon slots and superslots, a plurality of superslotsoccurring within each beacon slot.
 28. A communications device for usein a communications system where beacon time slots occur on a periodicbasis, a beacon signal being transmitted by a satellite base stationduring each beacon time slot according to a periodic downlink timingstructure, said downlink timing structure including a plurality ofsuperslots within each beacon slot, the individual superslots within abeacon slot being identifiable through the use of a superslot index,each superslot including a plurality of symbol transmission timeperiods, the device comprising: means for receiving at least one beaconsignal from the satellite base station; means for processing thereceived beacon signal to determine a downlink timing reference point,superslots occurring within a beacon slot having a predeterminedrelationship to the determined downlink timing reference point; meansfor transmitting an access probe signal to said base station; whereinsaid means for receiving are also for receiving a response to the accessprobe signal from the base station, the response including informationindicating at least one of: i) an amount of an indicated main superslottiming offset correction, the amount of the main superslot timing offsetcorrection being an integer multiple of a superslot time period; ii)superslot identifier indicating the position of a superslot within abeacon slot during which the base station received the access probesignal to which the received response corresponds; iii) identifier thatidentifies the communications device which transmitted the access probesignal to which the received response corresponds or iv) an identifierthat identifies the access probe to which the response being respondingto; and means for performing a transmission timing adjustment as afunction of the information included in the received response.
 29. Awireless terminal to communicate with base stations in a communicationssystem including a plurality of base stations, the wireless terminalcomprising: a memory for storing base station location information; areceiver configured to receive a signal from a first base station; alocation based timing determination module configured to determine atime at which to transmit an access signal to a second base station as afunction of stored location information indicating the location of saidfirst base station from which said wireless terminal received saidsignal; a transmitter configured to transmit said access signal at thedetermined time; wherein said location based timing determination moduledetermines a timing reference point from a signal received from thesecond base station, as part of determining said time at which totransmit an access signal to the second base station; wherein saidlocation based timing determination module determines the time at whichto transmit an access signal to the second base station relative to thedetermined timing reference point using uplink timing correctioninformation calculated from the stored location information indicatingthe location of said first base station and information indicating thelocation of said second base station, said time at which to transmit anaccess signal being determined so that the transmitted access signalwill be received at the second base station during an access interval inwhich received access signals are processed by said second base station.30. A wireless terminal to communicate with base stations in acommunications system including a plurality of base stations, thewireless terminal comprising: means for storing base station locationinformation; means for receiving a signal from a first base station;means for determining a time at which to transmit an access signal to asecond base station as a function of stored location informationindicating the location of said first base station from which saidwireless terminal received said signal; means for transmitting saidaccess signal at the determined time; wherein said means for determiningdetermine a timing reference point from a signal received from thesecond base station, as part of said determining the time at which totransmit an access signal to the second base station; and wherein saidmeans for determining determine the time at which to transmit an accesssignal to the second base station relative to the determined timingreference point using uplink timing correction information calculatedfrom the stored location information indicating the location of saidfirst base station and information indicating the location of saidsecond base station, said time at which to transmit an access signalbeing determined so that the transmitted access signal will be receivedat the second base station during an access interval in which receivedaccess signals are processed by said second base station.
 31. Acommunications terminal in an OFDM system including terrestrial andsatellite base stations which use OFDM signals for both uplink anddownlink signaling, the communications terminal comprising: a processorconfigured to control the communications terminal to: determine if abase station to which said communications terminal is seeking to sendOFDM uplink signals is a satellite base station or a terrestrial basestation; perform a first uplink timing synchronization process if it isdetermined that said base station is a satellite base station; andperform a second uplink timing synchronization process if it isdetermined that said base station is a terrestrial base station, saidsecond uplink timing synchronization process being different from saidfirst uplink timing synchronization process; wherein both saidterrestrial and satellite base stations have a downlink timing structurethat includes a plurality of superslots which recur in a periodicmanner, each superslot including a plurality of OFDM symbol transmissiontime periods; and wherein said first uplink tuning synchronizationprocess supports the communication of an uplink timing correction signalto the communications terminal which indicates an uplink transmissiontiming synchronization correction exceeding the duration of a downlinksuperslot.